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The Geological Society of America
Special Paper 557
Depositional and diagenetic history of travertine deposited within
the Anio Novus aqueduct of ancient Rome
Mayandi Sivaguru*,‡,†
Cytometry and Microscopy to Omics Facility, Roy J. Carver Biotechnology Center, University of Illinois at Urbana-Champaign,
Urbana, Illinois 61801, USA
Kyle W. Fouke‡,†
Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA
Duncan Keenan-Jones‡,†
School of Historical and Philosophical Inquiry, The University of Queensland, St Lucia QLD 4072, Australia
Davide Motta
Department of Mechanical and Construction Engineering, Northumbria University, Wynne Jones Building,
Newcastle upon Tyne, NE1 8ST, United Kingdom
Marcelo H. Garcia
Ven Te Chow Hydrosystems Laboratory, Department of Civil and Environmental Engineering,
Grainger College of Engineering, University of Illinois at Urbana-Champaign,
205 North Mathews Avenue, Urbana, Illinois 61801, USA
Bruce W. Fouke*,†
Department of Geology, School of Earth, Society and the Environment, University of Illinois at Urbana-Champaign,
Urbana, Illinois 61801, USA; Carl R. Woese Institute for Genomic Biology, University of Illinois at Urbana-Champaign,
Urbana, Illinois 61801, USA; Carle Illinois College of Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois
61801, USA; Roy J. Carver Biotechnology Center, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA;
and Department of Evolution, Ecology and Behavior, School of Integrative Biology, University of Illinois at Urbana-Champaign,
Urbana, Illinois 61801, USA
ABSTRACT
Travertine deposits preserved within ancient aqueduct channels record infor-
mation about the hydrology, temperature, and chemistry of the owing water from
which they precipitated. However, travertine is also chemically reactive and suscep-
tible to freshwater diagenesis, which can alter its original composition and impact
*Corresponding authors: sivaguru@illinois.edu, fouke@illinois.edu.
‡Joint rst authors.
†Authors contributed equally.
Sivaguru, M., Fouke, K.W., Keenan-Jones, D., Motta, D., Garcia, M.H., and Fouke, B.W., 2022, Depositional and diagenetic history of travertine deposited within
the Anio Novus aqueduct of ancient Rome, in Koeberl, C., Claeys, P., and Montanari, A., eds., From the Guajira Desert to the Apennines, and from Mediterranean
Microplates to the Mexican Killer Asteroid: Honoring the Career of Walter Alvarez: Geological Society of America Special Paper 557, p. 1–XXX, https://doi
.org/10.1130/2022.2557(26). © 2022 The Authors. Gold Open Access: This chapter is published under the terms of the CC-BY license and is available open access
on www.gsapubs.org.
OPEN ACCESS
G
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2 Sivaguru et al.
INTRODUCTION
A plentiful source of fresh drinking water, combined with
construction of an efcient and reliable water supply and distri-
bution system, have been the fundamental requirements needed
to sustain large centers of civilization throughout human history
(Scarborough, 1991). Ancient Rome still serves as the pinnacle
example of water supply infrastructure, where the largest and
most complex water delivery systems yet envisaged by human-
kind enabled population density to reach unprecedented levels
that rival those of modern-day urbanization (Morley, 1996).
Beginning in 312 B.C., 11 aqueducts were built to service Rome,
which in the end had 502 km of channels that were constructed
either below ground or elevated above ground atop arcades and
embankments depending on local topography and elevation
(Hodge, 2002a). These sprawling aqueduct systems were main-
tained at considerable expense for more than a millennium, which
testies to their importance to Rome’s elite (Coates-Stephens,
1998; Staccioli, 2007). Eventual expansion of the aqueduct
systems throughout the Roman Empire provided long-distance
water supply technology to Europe, North Africa, and the Near
East (Hodge, 2002a). In fact, Roman aqueducts were so effective
and durable that their basic design remained in widespread use
until the Industrial Revolution, and some Roman aqueducts are
still in use today (Keenan-Jones, 2013).
Ancient Rome brought unprecedented societal integration
and urban and economic development to an area that was large
enough to be comprised of a dozen modern-day nation-states.
One consequence was the dissemination of aqueduct technology
throughout this area, which resulted in the construction of more
than 2300 aqueducts (C. Passchier, 2018, ROMAQ: The Atlas
Project of Roman Aqueducts, https://www.romaq.org/; accessed
September 2020). Extensive aqueduct record keeping, completed
during the Republican and especially high Imperial periods, has
survived and provides detailed insight into the functioning of the
Eternal City’s aqueduct system. In particular, in A.D. 97, Sextus
Julius Frontinus was appointed Rome’s water commissioner and
compiled detailed (albeit sometimes internally inconsistent and
inaccurate) written descriptions (Frontinus, 2004) of the aque-
ducts and associated water distribution pipelines that supported
an estimated 600,000–1,000,000 people (Ashby, 1935; Keenan-
Jones, 2015). However, outside of Rome and even within Rome
after Frontinus, surviving records are patchy, particularly as the
empire and its capital faced ongoing monumental challenges. For
example, these included multiple outbreaks of disease such as the
Antonine plague (A.D. 166), the third-century A.D. military anar-
chy or “Imperial Crisis,” and repeated military incursions over the
next several centuries across the northern and eastern frontiers that
together caused the break-up of the Western Roman Empire in
the fth century A.D. (Heather, 2005). For the city of Rome, it
is not until the late medieval period (ca. A.D. 1300) that docu-
mentation exists rivaling that of the high empire a millennium
earlier, with ood records of the Tiber River. Therefore, much
remains unknown about this period, during which there was grad-
ual breakdown of the numerous aqueduct systems. This gap in
historical information has proven particularly vexing for scholars
reconstructions of aqueduct operation, maintenance, and climate. Hydraulic recon-
structions, in combination with a suite of high-resolution optical, laser, electron,
and X-ray microscopy analyses, have been used to determine the original crystal-
line structure and diagenetic alteration of travertine deposited in the Anio Novus
aqueduct built in A.D. 38–52 at Roma Vecchia. Age-equivalent travertine deposits,
precipitated directly on the mortar-covered oor at upstream and downstream sites
along a 140-m-long continuous section of the Anio Novus channel, exhibit consistent
crystalline textures and stratigraphic layering. This includes aggrading, prograd-
ing, and retrograding sets of travertine linguoid, sinuous, and hummocky crystal
growth ripples, as well as sand lags with coated siliciclastic grains deposited on the
lee slope of ripple crests. The original aqueduct travertine, which is similar to traver-
tine formed in analogous natural environments, is composed of shrub-like, dendriti-
cally branching aggregates of 1–3-μm-diameter euhedral calcite crystals. Dark brown
organic matter-rich laminae, formed by microbial biolms and plant debris, create
stratigraphic sequences of high-frequency, dark–light layering. This hydraulic and
petrographic evidence suggests that large, radiaxial calcites diagenetically replaced
the original aqueduct travertine shrubs, forming upward-branching replacement
crystals that crosscut the biolm laminae. While this diagenetic process destroyed the
original crystalline fabric of the calcite shrubs, the entombed biolm laminae were
mimetically preserved. These integrated approaches create the type of depositional
and diagenetic framework required for future chemostratigraphic analyses of trav-
ertine deposited in the Anio Novus and other ancient water conveyance and storage
systems around the world, from which ancient human activity and climatic change
can be more accurately reconstructed.
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 3
attempting to draw inferences from ancient Rome to evaluate the
possibilities of the future collapse of modern global society (Fer-
guson, 2010; Grinin et al., 2010; Yoffee and Cowgill, 1991).
An invaluable archive of information about human activity
and climatic change during these poorly documented periods
of history are the calcium carbonate (calcite, CaCO3) limestone
deposits that encrusted the interior oors, walls, and sometimes
even ceilings of aqueduct channels that extended throughout
the Roman Empire (Ashby, 1935; Hodge, 2002a, 2002b). Due
to their striking earthen colors and stratigraphic successions of
thin, dark–light laminations, these highly prized Roman aque-
duct limestone deposits were later quarried and polished for
use in church construction throughout Rome and other cities
(Ashby, 1935; Grewe, 1991). The geoscience classication of
similar CaCO3 limestone deposits precipitated in natural terres-
trial environmental settings includes the terms travertine (pre-
cipitates from high- to low-temperature springs, also called car-
bonate sinters), tufa (precipitates from low-temperature springs,
lakes, and waterfalls), and speleothem (precipitates from waters
in high- to low-temperature subterranean caves or fracture and
fault systems) (Sanders and Friedman, 1967). However, previ-
ous archaeological studies have used a wide variety of terms for
the limestone deposits that formed within Roman water supply
infrastructure, including travertine, tufa, calcareous crusts, calx,
calcium carbonate sinter, encrustation, lime, and lime scale (e.g.,
Aicher, 1995; Bobée et al., 2011; Brinker, 1986; Carlut, 2011;
Carlut et al., 2009; Carrara and Persia, 2001; Coates-Stephens,
2003a, 2003b, 2003c; Dubar, 2006a, 2006b; Garbrecht and Man-
derscheid, 1992; Gilly et al., 1971; Gilly, 1971; Hodge, 1992;
Lombardi, 2002; Schulz, 1986). In the present study, limestone
deposited from the water that owed within aqueducts will be
referred to as travertine (Fouke, 2011; Pentecost, 1995a, 2003,
2005; Pentecost and Viles, 1994; Sanders and Friedman, 1967).
Furthermore, the term travertine will be used here in a purely
descriptive sense that is solely determined by basic rock prop-
erties (e.g., crystalline structure, mineralogy, chemical composi-
tion, and stratigraphy) and without a priori interpretation of the
specic depositional or diagenetic environmental conditions in
which it was deposited.
Roman aqueduct travertine deposits primarily formed as
a result of mineral precipitation directly from the fast-owing,
chemically saturated waters being carried by the aqueducts. Only
a minor component of the travertine was formed by downstream
hydraulic transport of sedimentary particles and plant debris
within the aqueducts. Frontinus (Frontinus, 2004) and others in
ancient Rome mention aqueduct travertine accumulation as a
maintenance problem, but they do not give many details of how
it was handled (Fahlbusch, 1991; Frontinus, 2004; Leveau, 1991;
Taylor, 2000). Textual and physical evidence suggests that trav-
ertine was regularly removed by aquarii slaves working along
the aqueduct channels using hammers, chisels, spades, and per-
haps even vinegar, to prevent ow restriction within the aqueduct
channels (Bobée et al., 2011; Bruun, 1991; Coates-Stephens,
2003a, 2003b; Fahlbusch, 1991; Grewe and Blackman, 2001;
Leveau, 1991; Passchier, 2015; Taylor, 2000). However, not all
of the travertine was removed during cleaning, and it therefore
continued to accumulate from the owing water after mainte-
nance ceased and the aqueducts fell into neglect. Travertine pre-
served within aqueduct ruins therefore provides an invaluable
crystalline stratigraphic and geochemical record of the hydrol-
ogy and chemistry of the water that owed within ancient aque-
ducts throughout the Roman Empire (e.g., Ashby, 1935; Bobée
et al., 2011; Garbrecht and Manderscheid, 1992; Hostetter et al.,
2011; Lombardi et al., 2005; Passchier and Surmelihindi, 2019;
Passchier et al., 2021; Passchier et al., 2016a; Passchier et al.,
2016b; Passchier et al., 2020; Passchier and Sürmelihindi, 2010;
Passchier et al., 2013; Passchier et al., 2011; Puliti et al., 1986;
Sürmelihindi, 2013; Sürmelihindi, 2018; Sürmelihindi and Pass-
chier, 2013; Sürmelihindi et al., 2013a; Sürmelihindi et al., 2019;
Sürmelihindi et al., 2013b).
Over the past decade, a groundbreaking series of studies
have reconstructed paleoenvironmental conditions and archaeo-
logical information from the crystal growth, stratigraphic layer-
ing, and geochemistry of aqueduct travertine deposited in the
ancient Roman aqueducts of Italy, Turkey, France, and Jordan
(e.g., Hostetter et al., 2011; Passchier and Surmelihindi, 2019;
Passchier et al., 2021; Passchier et al., 2016a; Passchier et al.,
2016b; Passchier et al., 2020; Passchier et al., 2013; Sürmeli-
hindi, 2018; Sürmelihindi and Passchier, 2013; Sürmelihindi et
al., 2013a; Sürmelihindi et al., 2019; Sürmelihindi et al., 2013b).
In addition to containing stratigraphic sequences of dark–light
laminae, these aqueduct travertine deposits exhibit systematic
changes in their isotopic and trace element chemostratigraphy.
Coupled with changes in travertine crystal growth morphology
and evidence of human maintenance, this chemostratigraphic
evidence has been utilized to reconstruct annual air temperature,
rainfall, source water input, and Roman maintenance strate-
gies (e.g., Berking et al., 2018; Hostetter et al., 2011; Passchier
and Surmelihindi, 2019; Passchier et al., 2021; Passchier et al.,
2016a; Passchier et al., 2016b; Passchier et al., 2020; Passchier et
al., 2013; Sürmelihindi, 2018; Sürmelihindi and Passchier, 2013;
Sürmelihindi et al., 2013a; Sürmelihindi et al., 2019; Sürme-
lihindi et al., 2013b). At the same time, extensive research on
analogous travertine deposits in natural hot-spring, river, lake,
and cave systems around the world has shown that travertine
deposition results from complex, intertwined abiotic and biotic
processes (Andrews and Brasier, 2005; Della Porta, 2015; Fouke,
2011; Frisia, 2015; Pentecost, 1995b). The similarity in crystal-
line structure and composition of these natural travertine depos-
its suggests that aqueduct travertine also results from complex
and ever-changing physical, chemical, and biological processes
that were active at the time of deposition. Later diagenetic physi-
cal, chemical, and biological processes then serve to strongly
inuence the extent and products of post-depositional altera-
tion (Bathurst, 1975; Mcillreath and Morrow, 1990; Rodríguez-
Berriguete, 2020; Tucker and Bathurst, 1990; Tucker and Wright,
1990). However, to date, these understandings of travertine
deposition and diagenesis in natural systems have not been fully
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4 Sivaguru et al.
applied to determining the preservation and diagenetic alteration
of aqueduct travertine.
Of importance to the study of ancient aqueduct travertine
is the recognition that twenty-rst–century scientic explora-
tion is rapidly moving toward transdisciplinary approaches that
are capable of simultaneously characterizing physical, chemical,
and biological processes within complex natural and engineered
systems (National Research Council, 2014). This scientic inte-
gration is made possible by the ongoing development of new
technologies capable of simultaneously characterizing geologi-
cal and biological components comprising complex natural sys-
tems across broad spatial and temporal scales (U.S. Department
of Energy, 2009; Fouke, 2011; Wang et al., 2015). Called Geo-
BioMed when applied to human medicine, this approach has
recently been applied to reconstruct the crystallization and diage-
netic history of kidney stone formation (Saw et al., 2021; Sivag-
uru et al., 2020; Sivaguru et al., 2021a; Sivaguru et al., 2018a).
For example, standard optical brighteld (BF; transmitted light),
polarization (POL), and autouorescence (AF) thin section
microscopy has previously been conducted on aqueduct traver-
tine using standard research-grade petrographic microscopes with
a maximum spatial resolution of 3–5 m (Passchier et al., 2021).
However, microscopes developed for medical research have
recently been used on kidney stones to permit super -resolution
AF (SRAF) imaging at a spatial resolution of 140 nm (Saw et
al., 2021; Sivaguru et al., 2020; Sivaguru et al., 2021a; Sivaguru
et al., 2018a). This represents a more than ten-fold enhancement
in thin section optical and laser imaging resolution, which is
nearing resolutions attained with scanning electron microscope
(SEM) imaging. Another example is three-dimensional (3-D)
microcomputed tomography (microCT) X-ray analyses, which
now permit external surface rendering and non-destructive inter-
nal virtual cross-sectioning of whole hand sample solid crystal-
line deposits from millimeters to tens of centimeters in diameter
at a spatial resolution of 3 m (Saw et al., 2021; Sivaguru et al.,
2020; Sivaguru et al., 2021a; Sivaguru et al., 2018a). This repre-
sents an improvement of two orders of magnitude over standard
X-ray image resolution and offers expansive new opportunities to
understand the internal and external structure of solid crystalline
deposits such as kidney stones and aqueduct travertine. Applica-
tion of these next generation optical, laser, electron, and x-ray
imaging capabilities to human kidney stones have demonstrated
that they record multiple events of original crystal deposition that
are strongly impacted by repeated events of diagenetic alteration
(Saw et al., 2021; Sivaguru et al., 2020; Sivaguru et al., 2021a;
Sivaguru et al., 2018a).
The present study was undertaken to apply these Geo-
BioMed imaging and conceptual approaches to reconstruct-
ing the depositional and diagenetic history of travertine depos-
ited within the archaeologically signicant and protected ruins
of the Anio Novus aqueduct at Roma Vecchia (Fig. 1). Several
previous studies have described, measured, and documented the
structure, design, and architecture of the Anio Novus aqueduct
itself (Ashby, 1935; Mancioli and Sartorio, 2001; Reina et al.,
1917; Van Deman, 1934). Furthermore, the hydrology and con-
veyance of the Anio Novus has also been reconstructed from
the 3-D distribution and total thickness of travertine deposited
within the aqueduct channels (Keenan-Jones et al., 2015; Motta
et al., 2017). Taken together, these extensive previous studies
make the Anio Novus and its travertine deposits an exception-
ally well-suited archaeological laboratory within which to apply
GeoBioMed, high-resolution optical, laser, X-ray, and electron
microscopy. This establishes the resulting depositional and dia-
genetic reconstructions of the Anio Novus aqueduct travertine
as exemplars for studying travertine deposited in other ancient
water transport and storage systems around the world.
MATERIALS AND METHODS
Powers of Ten Contextual Framework
The study of travertine deposited within aqueducts is con-
fronted at the onset by challenges posed by scale, complexity,
and time (Anderson and Lewit, 1992; Goldenfeld et al., 2006).
To address these factors, a “Powers of Ten” conceptual frame-
work for aqueduct travertine was adopted (Fouke, 2011). This
approach recognizes that the primary components that make up
ancient aqueducts span length scales of 10−7 m to more than 102 m.
This nine-orders-of-magnitude dynamic range can be studied
with a combination of standard, well-established and next-gen-
eration microscope instrumentation, which in the present study
range from 140-nm-resolution SRAF microscopy to eld-based
observations and measurements of the upstream to downstream
sample sites along a 140 m run of the Anio Novus aqueduct chan-
nel (Fig. 1). This Powers of Ten framework for the study of Anio
Novus aqueduct travertine: (1) served to spatially and temporally
frame the experimental design and sampling strategy, which is
dictated by permit access, site conservation, and the number
and size of samples that can be collected; (2) determined the
choice and application of next-generation light, laser, electron,
and X-ray microscopy techniques; (3) permitted direct compari-
sons of the aqueduct travertine with travertine deposited in hot
springs, caves, and other natural environments; and (4) guided
the interpretation and discrimination of aqueduct travertine depo-
sitional and diagenetic crystalline fabrics.
Field Sample Collection
The largest and most signicant expansion of the water
supply system of ancient Rome commenced with construction
of the Claudia and Anio Novus aqueducts, which was started by
Emperor Caligula in A.D. 38 and nished by Emperor Claudius
in A.D. 52 (Fig. 1) (Aicher, 1995; Ashby, 1935; Hodge, 2002a).
The Anio Novus aqueduct, built directly on top of the Clau-
dia aqueduct (Figs. 1C–1D and 2A–2B), was the highest and
farthest-reaching of all 11 of Rome’s aqueducts. The Anio Novus
carried water over a distance of 87 km from its muddy and tur-
bid Aniene (Latin: Anio) River source near Subiaco (Fig. 1A),
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 5
Figure 1. Geographic setting of the Claudia and Anio Novus aqueducts of ancient Imperial Rome is shown. (A) Regional map shows
the course of the Anio Novus aqueduct from its source near Subiaco, through Roma Vecchia, and into downtown Rome. (B) Google
Earth image of the Claudia and Anio Novus aqueducts at Roma Vecchia shows the location of the upstream 0 m (sample RNRV3-
2A), intermediate, and downstream 140 m (sample RNRV1-2A) study sites. (C) Because the Anio Novus aqueduct at this location
is currently overgrown with foliage, this black and white photograph (modied from Van Deman et al., 1991) more clearly shows
the intermediate and 140 m sample sites as viewed from the southwest. (D) The Claudia and Anio Novus aqueducts north of the
140 m sample site are shown as viewed from the northeast.
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6 Sivaguru et al.
Figure 2. Photograph shows the downstream 140 m study site (sample RNRV1-2A) within the Anio Novus aqueduct at Roma Vec-
chia (geographic location is shown in Figs. 1B and 1C). (A) Field photograph of the 140 m sample site (white arrow), where the
walls of the ruin are partially preserved and the vaulted ceiling has been completely removed. (B) Cross-sectional sketch of the
140 m sample site (modied from Ashby 1935) illustrates travertine deposition on the oors and lower walls of the Anio Novus
and Claudia aqueduct channel. (C) The 140 m sample site (white arrow in A) is seen looking downstream along the aqueduct chan-
nel toward the northwest. The time-zero (t0) surface is the contact between the mortar of the underlying oor and the overlying
travertine. (D) Enlargement of white box shown in panel C. (E) Enlargement of white box shown in panel D. Travertine lee sands
(white arrow) deposited on the lee slope of ripple crests are easily visible in the eld. (F) Enlargement of hummocky ripples on the
uppermost surface of the 8-cm-thick layer of travertine deposited on the aqueduct channel oor (white arrow in panel D), which
was collected for analysis in the present study.
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 7
where a settling tank was built by the Romans between the river
and the aqueduct intake to help clean the water (Aicher, 1995;
Ashby, 1935; Hodge, 2002a). Constructed underground for most
of their length, the Claudia and Anio Novus aqueducts emerge
on raised embankments and arcades at Roma Vecchia (Figs.
1C–1D), which is 9 km southeast of Rome (Fig. 1A) (Aicher,
1995; Ashby, 1935; Hodge, 2002a). Here, the Anio Novus and
Claudia aqueduct ruins mark the beginning of an arcade, up to
32 m high, that brought the aqueducts into Rome’s city center
(Fig. 1A), where they are now a protected United Nations Edu-
cational, Scientic and Cultural Organization (UNESCO) World
Heritage Site. The waters of the Anio Novus were then mixed
with those of the Claudia aqueduct at the Vigna Belardi cistern
near Porta Maggiore in Rome. Maintenance of the Anio Novus
aqueduct likely ceased sometime between the fth and eighth
centuries A.D. (Coates-Stephens, 1998, 2004; Van Deman, 1934;
Van Deman et al., 1991).
Measurements of the thickness and distribution of traver-
tine deposits at multiple locations along the entire length of the
Anio Novus aqueduct channel have been published (Keenan-
Jones et al., 2014; Motta et al., 2017). The experimental design
of the present study was to collect and compare age-equivalent
upstream and downstream samples of travertine deposited on the
oor of a continuous 140-m-long section of the Anio Novus aque-
duct channel at Roma Vecchia (Figs. 1B–1C). Sample collection
was completed in March 2010 under research permits granted
by the Soprintendenza Speciale per il Colosseo, il Museo Nazio-
nale Romano e l’Area Archeologica di Roma. These included an
upstream 0 m site (sample RNRV3-2A) and a downstream 140 m
site (sample RNRV1-2A; Figs. 1B–1C). Travertine samples col-
lected from both the 0 m and 140 m sites (Figs. 1B–1C) were
composed of: (1) a 1-cm-thick uppermost portion of the underly-
ing Roman mortar lining the oor of the aqueduct; (2) the time-
zero (t0) surface at the contact between the underlying mortar and
the overlying travertine deposits; and (3) the 8-cm-thick layer of
aqueduct travertine deposited immediately above the t0 surface.
The 140 m site contains a 27-cm-thick section of travertine (Fig.
2C) (Keenan-Jones et al., 2015). However, only the lowermost
8 cm was sampled and studied at the 140 m site (Figs. 2C–2F)
because only the lowermost 8 cm of the travertine section was
preserved at the 0 m site. Age-equivalency of the 8-cm-thick trav-
ertine samples at both the 0 m and 140 m sites was established via
correlation to the t0 surface (Fig. 3).
At the 0 m site (Figs. 1B–1C), only the underlying support
arcade remained intact. Most original building materials com-
prising the Claudia aqueduct itself, as well as both the walls
and the vaulted ceiling of the Anio Novus aqueduct, have been
removed. Only a fallen, tilted block composed of the oor of the
Anio Novus aqueduct was preserved, and the 0 m site aqueduct
travertine sample was collected from the uppermost surface of
this block (Figs. 1C). Conversely, the Anio Novus aqueduct infra-
structure at the 140 m site (Figs. 1B–1C) was better preserved
(Figs. 2A–2C). Here, the supporting arcades and Claudia aque-
duct are intact, as are the oor and lower portions of the east wall
of the overlying Anio Novus aqueduct channel (Figs. 2A–2C).
The travertine deposits at both the 0 m and 140 m sites were par-
tially covered with soil and fully exposed to the elements (Figs.
1B–1C and 2C–2D).
Aqueduct travertine samples from the 0 m and 140 m sites
(Figs. 1B–1C and 2C–2D) were carefully removed using a ham-
mer and a small, clean, well-sharpened chisel composed of hard-
ened steel. Each travertine sample was labeled (sample number,
upstream-downstream context, and ow direction determined by
contextual orientation of the sample within the aqueduct chan-
nel), bagged, and shipped in a padded container to the Micros-
copy and Imaging Core Facility of the Carl R. Woese Institute
for Genomic Biology (IGB) at the University of Illinois Urbana-
Champaign, Illinois, USA. After X-ray analysis and orientation
(described below), samples were cut on a clean, water-cooled,
diamond-embedded tile saw in an orientation parallel to the
upstream-downstream ow direction of the channel. Samples
were then thoroughly washed with deionized water, polished,
and photographed with a Nikon SLR D7000 digital camera.
X-Ray Analyses
Two types of X-ray imaging were conducted on the aqueduct
travertine samples. The rst type was microcomputed tomogra-
phy (microCT) X-ray imaging completed on a North Star Imag-
ing X5000 system (Feinfocus 225 kV) at 63 m resolution at the
University of Texas High-Resolution X-Ray Computed Tomog-
raphy Facility, Austin, Texas, USA. The 3-D microCT data sets
were analyzed using either Imaris (Bitplane, Zurich, Switzer-
land) or Avizo (FEI, Thermosher, USA) 3-D rendering software
for both visualization and segmentation in the Microscopy and
Imaging Core Facility of the IGB. Several thousand microCT
images were compressed and converted into a maximum inten-
sity 3-D volume projection as virtual 5-mm-thick slices. These
virtual cross-section images were used to determine where to cut
5-mm-thick, vertical slices of the travertine hand samples par-
allel to the downstream ow direction on a diamond-embedded
tile saw for thin sectioning. The second type of X-ray imaging
was completed on a standard X-radiography system (Siemens
Model #10092624, 70 kV) at 150 m resolution in the Veterinary
School of Medicine at Illinois. Image compression, averaging,
gray-scale corrections, and line prole analyses were conducted
using open-source NIH ImageJ software. Averaged microCT
and X-radiography images were converted to TIFF les at both
8-bit and 16-bit gray scales for line transect analyses. Line prole
analyses were performed after the line widths were adjusted to
200 pixels to retrieve intensity peaks and valleys for location-
wise comparison of depositional stratigraphic patterns.
Petrographic Thin Section Preparation and
Optical Microscopy
Five billets cut from the travertine sample collected at the
0 m site and ve billets cut from the travertine sample collected
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8 Sivaguru et al.
Figure 3. Images show dark–light laminae stratigraphy of the Anio Novus aqueduct ripple-marked travertine samples
collected from the upstream 0 m site (sample RNRV3-2A; Figs. 1B and 1C) and the downstream 140 m site (sample
RNRV1-2A; Figs. 1B and 1C). Samples were correlated using the t0 contact surface between the underlying mortar and
the overlying travertine. All images were made from a 5-mm-thick, vertical cross-section slice oriented parallel to the
ow direction of the 0 m and 140 m samples, respectively. The downstream ow direction of the aqueduct water is in-
dicated on each image. Specic crystalline fabrics of the travertine samples are identied in Figures 4 and 5. (A and B)
Reected light photographs. (C and D) Standard X-radiography images. (E and F) Standard X-radiography images from
panels C and D are overlain on reected light photographs from panels A and B after pseudo-coloring and changes in
opacity, which show two distinct depositional units (Units 1 and 2) as well as signicant changes in the relative upstream
to downstream position, height, and wavelength of each ripple bedform.
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 9
from the 140 m site (Figs. 1B–1C) were prepared by Wagner
Petrographic (Linden, Utah, USA) as Petropoxy impregnated,
doubly polished, uncovered, 25-m-thick sections mounted on
standard-sized petrographic glass slides. Optical microscopy of
these thin sections was completed on multiple microscopes, all
of which are housed in the Illinois IGB Microscopy and Imag-
ing Core Facility and have been previously described in detail
(Sivaguru et al., 2014a; Sivaguru et al., 2019a; Sivaguru et al.,
2014b; Sivaguru et al., 2019b; Sivaguru et al., 2020; Sivaguru et
al., 2012; Sivaguru et al., 2018a; Sivaguru et al., 2021b; Sivaguru
et al., 2018b). In brief, modalities included bright-eld (BF),
polarization (POL), phase-contrast (PC), cathodoluminescence
(CL), and wide-eld and confocal auto-uorescence (AF) with
merged pseudo-colored red, green, and blue (RGB) channels
using DAPI, FITC, and Rhodamine lters. Analyses were done
across a broad range of magnications (10×: 0.3 numerical aper-
ture (NA); 20×: 0.8 NA; 63×: 1.4 NA; and 100×: 1.46 NA) with
Plan Neouar (10×), Plan Apochromat (20–63×), and Alpha Plan
Apochromat (1.46 NA) objectives.
BF, POL, PC, wide-eld, and confocal (LSM 700, Carl
Zeiss, Oberkochen, Germany) AF microscopy was conducted at
a resolution of 250 nm on a ZEISS Axio Observer Z1 system
with 20× (0.8 NA) Plan Apochromat and 50× (0.95 NA) Plan
Neouar POL objectives for both brighteld (BF) and polariza-
tion (POL) microscopy. In addition, CL microscopy was con-
ducted on a custom-built Reliotron cathodoluminoscope stage
(RELION Industries LLC, Bedford, Massachusetts, USA) oper-
ating at 11 kV and 550 A and mounted on a ZEISS AxioZoom.
V16. The same system has also been used to collect BF, POL, and
AF images from thin sections using either a DL 450 LED white
light source base or an X-Cite metal halide mercury uorescent
lamp (for AF images) with a ZEISS Axiocam 512 color camera
for imaging (Carl Zeiss, Oberkochen, Germany). All microscopy
images were processed using ZEISS ZEN Blue software. Red,
green, and blue (RGB) curves for each image were adjusted and
presented as linear or with a gamma adjustment of 0.4–0.5, min/
max, best mode or manually adjusted in the display properties
window in the ZEISS ZEN software for representative brightness,
contrast, and clarity. Images obtained from the Avizo, Imaris, and
ZEN programs were compiled in Adobe Photoshop and Adobe
Illustrator after further intensity and size adjustments as required
(Adobe Inc., San Jose, California, USA). These instruments and
accompanying image analysis software workstations are housed
in the Illinois IGB Microscopy Core Image Analysis Facility.
BF, POL, and AF microscopy was also completed on a
ZEISS Axio Scan.Z1 whole slide scanning system (with 20×
[0.8 NA] Plan Apochromat and 50× [0.95 NA] Plan Neouor
POL objective) and a ZEISS Axio Zoom.V16 microscope (with
1.0× [NA 0.25] Plan Apochromat objectives; Carl Zeiss Com-
pany, Oberkochen, Germany). Thin sections were illuminated
with a DL 450 LED light source base and imaged with a ZEISS
Axiocam 512 color camera (Carl Zeiss, Oberkochen, Germany).
Travertine AF from thin sections was then further analyzed
using a ZEISS LSM 880 Confocal Laser Scanning Microscope
with Airyscan SRAF (Carl Zeiss, Oberkochen, Germany) as
described previously (Sivaguru et al., 2019a; Sivaguru et al.,
2014b; Sivaguru et al., 2020; Sivaguru et al., 2012; Sivaguru et
al., 2018a; Sivaguru et al., 2021b; Sivaguru et al., 2018b). Exci-
tation and emission wavelengths that were collected included:
405 nm excitation (emission collected between 410 nm and
460 nm), 488 nm excitation (emission collected between
500 nm and 550 nm), and 561 nm excitation (emission collected
between 570 nm and 615 nm).
Environmental Scanning Electron Microscopy (ESEM)
Small 1 cm3 cubes were cut from the Anio Novus aqueduct
travertine. These were washed, air dried, and attached to a sample
holder stub (Z1506P, SPI Supplies West Chester, Pennsylvania,
USA) using double-sided carbon tape (cat. no. Z05073, SPI Sup-
plies). These samples were then sputter-coated with an ~60-nm-
thick layer of gold–palladium (Desk II TSC sputter coater,
Denton Vacuum, Moorestown, New Jersey, USA). Each sample
was imaged using an environmental scanning electron micro-
scope (ESEM) in high-vacuum mode (Quanta 450 FEG ESEM,
Thermo Fisher FEI, Hillsboro, Oregon, USA) at 5–10 kV and
multiple magnications, housed in the Microscopy Suite of the
Imaging Technology Group in the Illinois Beckman Institute for
Advanced Science and Technology, Illinois, USA.
RESULTS
Depositional Units and Crystal Growth Ripples
Standard reected light photography, combined with high-
resolution optical, laser, electron, and X-ray microscopy, indi-
cates that the Anio Novus aqueduct travertine deposited at both
the upstream 0 m and downstream 140 m sites (Figs. 1B–1C) is
composed of two depositional horizons herein called Units 1 and
2 (Figs. 3–5). Unit 1 (the lowermost 5 cm section) and Unit 2 (the
uppermost 3 cm section), which were age-correlated by means of
the t0 surface, are strikingly similar in color, crystalline texture,
and stratigraphic layering at both the 0 m and 140 m sites (Figs.
3–6). Unit 1, deposited directly on the t0 surface, is composed
of high-frequency interlayering of 0.1–1-mm-thick, dark brown
laminae and light beige laminae (Figs. 3–6). Laminae within the
basal 1 cm of Unit 1 at the 0 m site are nearly planar and hori-
zontal (Figs. 3–6). Conversely, at the 140 m site, the stratigraphic
layering within Unit 1 exhibits an angular unconformity (Boggs,
2012) where underlying horizontal beds are eroded and overlain
by a sequence of inclined laminae (Figs. 4B and 5B). Unit 1 stra-
tigraphy above the 1-cm-thick basal interval exhibits up-section
increasingly larger and more pronounced travertine crystal growth
ripple morphologies (i.e., stoss, crest, lee, and trough; Figs. 3–6)
(Keenan-Jones et al., 2022). On bedding surfaces observed in
the eld and hand samples (Figs. 4C–4D), as well as virtual 3-D
micro-CT horizontal sections (Figs. 5C), these bedforms have
been identied as linguoid and sinuous ripples (Boggs, 2012). As
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10 Sivaguru et al.
these ripple sets vertically accumulated, they shifted from down-
stream progradation to upstream retrogradation, which created
zig-zag stratigraphic geometries in vertical cross-section (Figs.
3–6). A bedding surface observed within Unit 1 at a location 9 m
downstream from the 0 m sample site (Figs. 1B–1C) also exhibits
hummocky ripples (Fig. 4D).
In contrast, both the aqueduct travertine crystal growth rip-
ples and laminae stratigraphy change dramatically in Unit 2 (Figs.
3–6). While the thicknesses of the dark brown laminae remain
relatively consistent between Units 1 and 2 (hundreds of µm thick
to ~1 mm thick), the light beige laminae become signicantly
thicker in Unit 2 (~1–3 µm thick; Figs. 3–6). Bedforms observed
Figure 4. Standard reected light hand
sample photographs show Anio Novus
aqueduct ripple-marked travertine de-
posits. (A and B) Vertical cross-sections
(oriented parallel to the aqueduct chan-
nel ow direction) of travertine samples
collected from the upstream 0 m site
(sample RNRV3-2A; Figs. 1B and 1C)
and the downstream 140 m site (sample
RNRV1-2A; Figs. 1B and 1C). Micro-
computed tomography (microCT) im-
ages of the same samples (prior to cut-
ting) are shown in Figure 5. Example lee
sand lag deposits are shown with black
arrows, and example radiaxial calcite
crystals are shown with blue arrows. In
the 0 m travertine sample in image B,
an angular unconformity occurs near the
base of Unit 1 as indicated by red arrows
(also visible in the microCT scan of the
same location in Fig. 5). Depositional
Units 1 and 2 are also shown. (C) Field
photograph of the uppermost surface of
a travertine hand sample collected at the
0 m sample site, which exhibits linguoid
ripples (stratigraphic position is shown
in image A with a C). (D) Uppermost
surface of a channel travertine hand
sample, collected at the intermediate
site (Figs. 1B and 1C), which exhibits
sinuous ripples (stratigraphic position
shown in image A with a D).
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 11
in hand sample (Fig. 2F) and virtual horizontal microCT cross
sections (Fig. 5D) indicate that Unit 2 is composed of sinuous
and some hummocky ripples (Boggs, 2012). Like the ripples in
Unit 1, the aggrading crests of the ripple sets in Unit 2 exhibit
progradation and retrogradation, which create similar zig-zag
stratigraphic patterns in vertical cross-section (Figs. 3–6). How-
ever, these stratal patterns are less obvious in Unit 2 because there
are fewer lag deposits leeward of the ripple crests due to the lon-
ger ripple wavelengths (Figs. 3–6).
Lee Sands within Ripple Sets
A common depositional feature of the Unit 1 and 2 linguoid
and sinuous ripple-marked travertine is the accumulation of
Figure 5. Microcomputed tomography
(microCT) images of Anio Novus aq-
ueduct travertine deposits are shown.
Each section represents a single vertical
virtual microCT slice from over 3700
virtual microCT slices collected from
each hand sample. (A and B) Vertical
cross-sections (oriented parallel to the
aqueduct channel ow direction) are
shown of travertine collected from the
upstream 0 m site (sample RNRV3-2A;
Figs. 1B and 1C) and the downstream
140 m site (sample RNRV1-2A; Figs.
1B and 1C). Reected light hand sam-
ple photographic images of the same
samples (after cutting) are shown in
Figure 4. Example lee sand lag deposits
are shown with black arrows; upward-
radiating radiaxial calcite crystals are
shown with blue arrows, and the angu-
lar unconformity near the base of Unit 1
in the 0 m site sample is indicated with
red arrows. Depositional units (Units 1
and 2) are also shown. (C) Virtual hori-
zontal microCT slice collected from the
stratigraphic position shown in image
A with a C, which documents linguoid
ripple marks in the travertine that are
consistent with those observed in hand
sample (Fig. 4C). (D) Virtual horizontal
microCT slice collected from the strati-
graphic position shown in image A with
a D, which shows sinuous ripple marks
in the channel travertine that are consis-
tent with those observed in hand sample
(Fig. 4D).
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12 Sivaguru et al.
Figure 6. High-resolution bright eld
(BF) microscopy of a thin section from
the Anio Novus Unit 1 aqueduct traver-
tine sample collected from the upstream
0 m site (sample RNRV3-2A; Figs. 3–5).
Images A, B, and C exhibit abundant
dark brown laminae that are laterally
continuous and therefore age-equivalent
in the travertine shrubs (TS), where they
are more diffuse, and within radiaxial
calcites (RC), where they become ner,
well-dened, and show multiple lami-
nae. (A) Middle section of Unit 2 traver-
tine deposit with sinuous ripples that ex-
hibit lateral continuity of age-equivalent
laminae within the TS and RC. (B) Up-
per section of Unit 1 travertine exhibits
linguoid and sinuous ripples in vertical
cross-section and shows TS, RC, and
regions in which lee sands (LS) were
plucked during hand sample cutting and
thin section preparation. (C) Lowermost
section of Unit 1 travertine deposit with
planar laminae deposited on the time-
zero (t0) surface, which become small-
scale linguoid ripples.
siliciclastic grains on the lee slope of ripple crests, herein called
lee sands (Fig. 7) (Keenan-Jones et al., 2022). These lag deposits
contain an assortment of ne to coarse and angular to rounded
siliciclastic sands, which were densely packed on the lee slope of
each linguoid ripple. These lee sands are well-documented in 3-D
microCT imaging of the aqueduct hand samples prior to sawing
and thin section preparation (Figs. 7D–7F). However, the process
of cutting and sectioning plucked and removed the majority of
these siliciclastic sand grains (Figs. 7A–7C). The remaining void
spaces exhibit periodic bridging by continuous layers of euhe-
dral calcite crystal cements, in which some coated grains became
encrusted and suspended (Figs. 7A–7C). These age-equivalent,
calcite-cemented depositional layers can be continuously traced
downstream across the ripple crest, through the lee sands, and
into the ripple trough (Fig. 7C). Furthermore, the lee sand con-
centrations geometrically track the vertical aggradation of the
stratigraphic ripple sets (Figs. 7D–7F), which serve to accentuate
the zigzag stratal geometries formed during downstream progra-
dation and upstream retrogradation (Figs. 3 and 7). Lee sands are
also found on ripple lee slopes in Unit 2 (Figs. 3 and 7). However,
there are fewer lee sands deposited in Unit 2 than in Unit 1 (Figs.
3 and 7), which correlates with longer ripple wavelengths and
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 13
Figure 7. High-resolution optical and X-ray microscopy images show lee sand deposits on the lee slope of linguoid ripple
crests of a thin section from the Anio Novus aqueduct sample collected from Unit 1 at the upstream 0 m sample site
(sample RNRV3-2A; Figs 1B and 1C and 3–5). (A) Reected light, extended depth photograph overlain with an autouo-
rescence (AF) image of the vertical face of travertine slices oriented parallel to the ow direction shown in Figures 3A
and 3B. This merger of images causes the dark brown biolm laminae to appear pink to red. As the travertine vertically
aggrades, the ripple sets slightly prograde and retrograde, creating the inclined and zig-zag geometries of the lee sands
(LS) deposited on the lee slope of ripple crests. Some grains were plucked during sample cutting and thin section prepara-
tion. (B–C) Bright eld (BF) microscopy imaging shows travertine shrubs (TS), radiaxial calcite (RC), and coated grains
(CG) that comprise the lee sand lags. (D–F) Microcomputed tomography (microCT) virtual vertical cross-section images
of travertine Unit 1 prior to cutting and thin sectioning. This conrms that regions with lee sand lags (bright white grains
in A) were completely lled with sedimentary grains, the coarsest of which (shown in mauve in image E and in gray in
image F) were deposited when the linguoid and sinuous ripples begin to prograde downstream.
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14 Sivaguru et al.
Figure 8. High-resolution bright eld (BF), superresolution autouorescence (SRAF), and electron microscopy images of
thin sections from the Anio Novus aqueduct travertine sample collected from the upstream 0 m site (sample RNRV3-2A;
Figs. 3–5) are shown. These crystalline fabrics and textures were observed throughout depositional Units 1 and 2 (Figs.
3–5). In all images, age-equivalent dark brown laminae are more diffuse within travertine shrubs (TS) and become mul-
tiple, ner, well-dened laminae in radiaxial calcites (RC). (A–C) TS composed of dendritically branching aggregates
of euhedral calcite crystals imaged with (A) BF, (B) polarized light (POL), and (C) environmental scanning electron
microcopy (ESEM). (D–F) Lateral and vertical contacts between TS and RC imaged with (D) BF, (E) ring aperture con-
trast (RAC), and (F) POL. White arrows track age-equivalent dark laminae from being diffuse in TS to forming multiple
ne laminae in RC. (G) Superresolution autouorescence (SRAF) microscopy (140 nm resolution) at the lateral contact
between TS and RC. White arrows track age-equivalent dark laminae from being diffuse in TS to forming multiple ne
laminae in RC. The travertine sample was not impregnated with epoxy, which conrms that the AF emissions emanating
from the travertine are not derived from epoxy. This image was created by merging red, green, and blue channels of AF.
(H) Environmental scanning electron microscope (ESEM) image of a vertical contact between TS and RC from an etched
and polished hand sample.
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 15
results in the formation of fewer ripple crests and therefore fewer
lee sand lag deposits.
Crystalline Shrubs and Laminae
The term “spar” has not been used in this study, because
the term is often broadly linked with interpretations of specic
environments of crystallization (Folk, 1959, 1962). Instead, the
term “crystal” will be used in its place as a purely descriptive
term (Dunham, 1962; Tucker and Wright, 1990). The Unit 1
and 2 aqueduct travertine deposits are composed of two types
of crystalline growth morphologies, which are described here as
“shrubs” (Fouke et al., 2000) and “radiaxial calcites” (Kendall,
1977; Kendall and Broughton, 1977; Tucker and Kendall, 1973).
Both types of these crystalline deposits contain stratigraphic
sequences of interlayered dark brown and light beige laminae
(Figs. 3–7). The travertine shrubs are composed of 100-m-tall,
dendritically branching aggregates of small (1–3-m-diameter)
euhedral calcite crystals (Figs. 8A–8C). The travertine radiaxial
calcites are composed of variably sized (hundreds of microns to
tens of millimeters), upward-radiating and branching crystalline
bundles that crosscut the alternating dark–light laminae stratigra-
phy (Figs. 3–7). The radiaxial calcites are composed of elongated
crystals with irregular intercrystalline boundaries, subcrystals
that diverge away from the substrate, a convergent length-fast
structure, and twin lamellae (Figs. 3–7). These growth forms
are consistent with radiaxial calcites previously reported from
multiple types of natural limestones throughout the geological
record (Kendall, 1985; Kendall and Broughton, 1978; Kendall
and Dunn, 1977; Tucker and Kendall, 1973). Vertical and lateral
contacts between the travertine shrubs and radiaxial calcite are
irregular (Figs. 8D–8H) and composed of individual travertine
shrub calcite crystals that are partially engulfed and entombed
within large clear radiaxial calcite crystals (Figs. 8D–8H). These
dark brown laminae extend undisturbed laterally through the
travertine shrubs and radiaxial calcites (Figs. 8D–8H). Under
BF microscopy, these dark brown laminae form thicker and
more seemingly diffuse tens-to-hundreds-of-m-thick, dark
brown laminae within the travertine shrubs (Figs. 8D–8G). The
laminae then laterally transition into multiple signicantly ner
(1–3-m-thick) and more distinct brown laminae within the radi-
axial calcite crystals (Figs. 8D–8G). These dark brown laminae
in both the travertine shrubs and radiaxial calcites exhibit bright
green AF and bright orange CL emissions, which are signi-
cantly brighter within the radiaxial calcites (Figs. 9 and 10).
DISCUSSION
High-resolution optical, laser, electron, and X-ray micros-
copy analyses were conducted in the present study to characterize
the crystalline structure, bedform geometries, and laminae stratig-
raphy of the travertine deposited within the Anio Novus aqueduct.
From these analyses, the depositional histories of the aqueduct
travertine crystalline shrubs and their laminae are discussed within
the context of the hydraulic setting of the Anio Novus, modern-
day aqueous chemistry of the Aniene River source water, and
comparisons are made to other analogous ancient aqueduct and
natural systems. The diagenetic history of the Anio Novus aque-
duct travertine is then evaluated using the same high- resolution
microscopy analyses, which suggest that the abundant radiaxial
calcite crystals are diagenetic replacement products of the original
travertine. This historical framework of deposition and diagenesis
is then used to evaluate the potential for future reconstructions
of human history and paleoclimate from travertine deposits pre-
served within water conveyance and storage infrastructure built
by ancient civilizations around the world.
Depositional History of the Anio Novus
Aqueduct Travertine
Depositional Age
While the absolute age of the 8-cm-thick travertine depos-
ited on the t0 channel oor surface at the 0 m and 140 m sites
at Roma Vecchia (Figs. 1B–1C) remains uncertain, circumstan-
tial evidence implies that the travertine may have been deposited
relatively soon after construction of the Anio Novus aqueduct.
Because the t0 surface cannot be older than the A.D. 38–52 con-
struction period of the Anio Novus itself (Ashby, 1935), pos-
sible depositional ages include: (1) the entire Roman Imperial
period (A.D. 30–476) (Stevenson, 2009), when the Anio Novus
was actively owing, used, and maintained (Coates-Stephens,
2003a, 2003b), and (2) much later into early and middle medi-
eval times (ca. A.D. 477–1000) (Stevenson, 2009), when the
Anio Novus still functioned to transport water but was no longer
maintained (Coates-Stephens, 2003a, 2003b). However, there
is no evidence of rebuilding of the channel or its mortar lining
after the Anio Novus aqueduct’s initial construction at Roma
Vecchia (Ashby, 1935). Furthermore, common Roman cleaning
techniques included physical chiseling, scraping, and perhaps
dissolution from the application of heated wine vinegar (Bruun,
1991; Fahlbusch, 1991; Grewe and Blackman, 2001; Leveau,
1991; Taylor, 2000). As a result, the lack of gouging or scraping
along the t0 surface implies that the Anio Novus had not been
removed during maintenance prior to deposition of the basal,
8-cm-thick travertine deposits at the 0 m and 140 m sites. There
is also no evidence that a waterproof mortar lining was placed
over the encrusted aqueduct travertine, which has been com-
monly documented in other ancient Roman aqueducts (Passchier
and Surmelihindi, 2019; Passchier et al., 2021; Porath, 2002;
Sürmelihindi, 2013). This extra thick mortar was routinely added
to further protect the vulnerable corner contacts of the oor and
wall (Hodge, 2002b). Frontinus also reported that this additional
aqueduct masonry served as structural reinforcement to support
the immense weight of the travertine precipitated within some
aqueducts. However, the absence of these cleaning and mainte-
nance features, combined with the perfectly smooth and planar
nature of the t0 surface, implies that the lowermost 8 cm of Anio
Novus travertine was deposited relatively soon after construction
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16 Sivaguru et al.
Figure 9. High-resolution bright eld
(BF), autouorescence (AF), and cath-
odoluminescence (CL) microscopy im-
ages of thin sections from the Anio No-
vus aqueduct travertine sample collect-
ed from the upstream 0 m site (sample
RNRV3-2A; Figs. 3–5) are shown. All
of these crystalline fabrics and textures
were observed throughout both Units 1
and 2 (Figs. 3–5). White arrows in imag-
es B through F show dark brown lami-
nae that is more diffuse within travertine
shrubs (TS) and becomes multiple, ner,
well-dened, and age-equivalent lami-
nae in radiaxial calcite (RC). (A–C) (A)
BF, (B) AF, and (C) CL images were all
made from the same position on the thin
section and document the replacement
of RC with TS. Note that dark laminae
within RC exhibit brighter orange CL
than age-equivalent laminae within TS
(tracked with white arrows). (D–F) (D)
BF, (E) AF, and (F) CL images were all
taken from the same position on the thin
section at ripple lee grains (LG) and
document the replacement of RC with
TS. Note that dark laminae within RC
exhibit brighter orange CL than age-
equivalent laminae within TS (tracked
with white arrows).
and not subjected to maintenance cleaning prior to deposition of
the travertine analyzed in the present study.
Water Temperature and Chemistry
The water temperature and chemistry of the upper Aniene
River watershed, which is where the Subiaco water intake of the
Anio Novus aqueduct was located, serves as a valuable mod-
ern analog for the ancient owing water that was transported
through the Anio Novus aqueduct (Bono and Boni, 1996; Bono
et al., 2001; Bono and Percopo, 1996). The Aniene River basin
drains the karsted Mesozoic carbonate lithologies of the Simbru-
ini Mountains in the Lazio Region of the Appenines (Bono and
Percopo, 1996). A gauging station at Subiaco has long provided
measurements of modern-day temperature, ow discharge, and
suspended solid load of the Aniene River under both normal and
storm atmospheric conditions (Bono and Percopo, 1996). As a
result, ancient Aniene River water, supersaturated with respect to
calcite, is thought to have entered the Anio Novus intake at tem-
peratures of 10–15 °C and a pH of 6.7–7.2, with aqueous chemi-
cal compositions typical of karsted limestone terrains (Bono and
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 17
Boni, 1996). Furthermore, calcite precipitation rates, aqueous
chemistry, and ow conditions have been quantied for the Tar-
tare karst spring near Subiaco (Bono et al., 2001). These modern-
day Aniene River watershed aqueous analyses, combined with
ancient conveyance rates reconstructed from the thickness of
travertine deposited on the oor, walls, and ceiling of the Anio
Novus (Keenan-Jones et al., 2015; Motta et al., 2017), suggest
that the ancient owing Anio Novus aqueduct water at Roma
Vecchia remained supersaturated with respect to calcite at 10–15
°C and a pH of 6.7–7.2 by the time it reached Roma Vecchia
(Bono and Boni, 1996).
Hydraulic Conditions
When the Anio Novus was in operation, travertine accumu-
lations on the oor, walls, and ceiling of the aqueduct channel
at Roma Vecchia would have reduced the cross-sectional area of
the aqueduct channel (Keenan-Jones et al., 2014; Motta et al.,
2017) Aqueduct water ow velocity under these restricted condi-
tions is estimated to have varied by more than 1 m/s from place
to place along the 80-km-length of the Anio Novus due to signi-
cant changes in the amount of travertine deposition and hillslope
topography (Keenan-Jones et al., 2015; Motta et al., 2017). Aver-
age ow velocity estimates for the entire Anio Novus range from
0.7 m/s to 0.9 m/s (Bono and Boni, 1996; Fahlbusch, 1987). Addi-
tionally, it has recently been estimated that full gravity ow of the
Anio Novus aqueduct was 1.2 ± 0.4 m3/s, which has been recon-
structed from the wetted perimeter of the outermost surface of the
travertine at Roma Vecchia (Keenan-Jones et al., 2014, 2022).
Hydraulic reconstructions have also been made from quan-
titative analyses of the wavelength, amplitude, and steepness of
the Anio Novus travertine linguoid, sinuous, and hummocky
crystal growth ripples at Roma Vecchia (Kennan-Jones et al.,
2022). Taken together, these measurements suggest that the
critical shear Reynolds number of channel ow at the time of
travertine deposition was equivalent to being much higher than
those calculated from bedforms measured in caves, ice, and u-
vial and marine siliciclastic systems (Keenan-Jones et al., 2015,
2022; Motta et al., 2017). In addition, the high-velocity conned
channel ow of the Anio Novus at Roma Vecchia occurred at
virtually constant kinematic viscosity, where ripple wavelength
decreased with increased shear velocity. These extremely high
ow rates would have decreased the thickness of the boundary
layer of the base of the owing water on the upper surface of the
growing aqueduct travertine. In addition, surface scouring (sand
blasting) would have taken place on the upper surface of the
travertine by the lee sands that were deposited and preserved on
the lee side of travertine crystal growth ripples (Fig. 7) (Keenan-
Jones et al., 2022).
Figure 10. High-resolution bright eld
(BF) and cathodoluminescence (CL)
microscopy images of thin sections
from Anio Novus aqueduct samples
collected from the upstream 0 m site
(sample RNRV3-2A; Figs. 3–5) and the
downstream 140 m site (sample RN-
RV1-2A; Figs. 3–5) are shown. Sample
site locations are shown in Figures 1B
and 1C. CL images are overlain on the
BF images, which illustrates that dark
laminae within radiaxial calcites (RC)
exhibit brighter orange CL than age-
equivalent laminae within travertine
shrubs (TS). (A and D) Thin section
from the middle of Unit 2 travertine. (B
and E) Thin section from the middle of
Unit 1 travertine. (C and F) Thin section
from the base of Unit 1 travertine, which
shows a t0 contact.
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18 Sivaguru et al.
The combined effects of these hydraulic factors would
have acted to prevent dendritically branching crystalline struc-
tures, such as the upward radiating bundles of radiaxial calcites
observed in hand samples and with standard X-radiography and
microCT (Figs. 3–6), from growing on the channel oor. This
is further indicated by the dense mounded and rippled surfaces
seen in hand sample and outcrop, none of which exhibits the
terminations of large crystalline shrubs or other crystal growth
faces (Figs. 2F, 3C–3D, and 4C–4D). This high-velocity ow
regime may also explain the extremely small size of the aque-
duct’s travertine crystalline shrubs (Fig. 7) relative to the size of
crystalline shrubs observed in analogous lower velocity natural
systems (Della Porta, 2015). Importantly, these hydraulic con-
straints would also have held true for other ancient aqueducts
throughout the Roman Empire in France, Turkey, and Jordan,
in which travertine crystal growth ripples are well-preserved
(Passchier and Surmelihindi, 2019; Passchier et al., 2021;
Passchier et al., 2016a; Passchier et al., 2016b; Passchier et al.,
2013; Sürmelihindi, 2018; Sürmelihindi and Passchier, 2013;
Sürmelihindi et al., 2013a; Sürmelihindi et al., 2019; Sürmeli-
hindi et al., 2013b).
Depositional Units and Ripple Lee Sands
Correlation of the t0 surface and strong similarities in trav-
ertine color, depositional texture, and stratigraphic layering pat-
terns (Figs. 3–6) suggest that Units 1 and 2 deposited at the 0 m
and 140 m sample sites are equivalent in depositional age. Out-
crop bedding surfaces, dark–light laminae stratigraphy, and
microCT imaging in the present study (Figs. 3–6) indicate that
Unit 1 is composed of aggrading linguoid and sinuous traver-
tine crystal growth ripples, which exhibit multiple episodes of
downstream progradation and upstream retrogradation as they
vertically aggraded. In addition, the striking angular unconfor-
mity observed near the bottom of Unit 1 in the 140 m sample
(Fig. 4B) indicates that some type of erosion took place, either
due to changing ow conditions or possibly Roman maintenance.
In contrast, Unit 2 is composed of sinuous ripples that aggraded
vertically and exhibit lesser degrees of downstream progradation
and upstream retrogradation as well as hummocky ripples.
The Unit 1 travertine linguoid and sinuous crystal growth
ripples also exhibit lee sand lag deposits composed of poorly
sorted siliciclastic grains (Fig. 7). These lee sands, transported
and deposited during the ongoing high-ow conditions of the
Anio Novus waters, would have been rotated and rolled by Kár-
mán spiral eddies (vortices) leeward of each linguoid ripple crest
(Fig. 6). At the same time, small calcite cements encrusted the
outer surfaces of these siliciclastic sands to form coated grains
(Bathurst, 1975; Mcillreath and Morrow, 1990; Tucker and
Bathurst, 1990; Tucker and Wright, 1990). Depositional lami-
nae laterally extended downstream from the linguoid ripple stoss
slopes, through the ripple crests, lee slopes, and troughs (Figs.
7 and 9). Ashby (Ashby, 1935) and Blanco and Sebastiani del
Grande (Blanco and Sebastiani del Grande, 2016) were likely
referring to these coated grains when describing “calcareous peb-
bles, completely round” that lled the settling tank in the Anio
Novus at Villa Bertone (700 m upstream of Roma Vecchia). It is
also possible that the high-velocity aqueduct waters were capable
of transporting some of these siliciclastic sands from the Subiaco
source area (Fig. 1). Conversely, although to our knowledge not
previously suggested in the literature, it may also be possible
(but unproven here) that these siliciclastic grains were periodi-
cally added by the Romans at access points upstream of the 0 m
and 140 m sites in an attempt to “sand blast” and clear travertine
deposits from within the channel of the Anio Novus.
Crystalline Shrubs
The high-resolution microscopy analyses conducted in the
present study indicate that the Anio Novus aqueduct traver tine was
originally composed of crystalline shrubs, which were made of
dendritically branching aggregates of small (1–3-m-diameter),
euhedral calcite crystals (Figs. 6, 8, and 9). These syn-deposi-
tional shrubs grew with dark brown laminae that formed diffuse
layers tens to hundreds of microns thick (Figs. 6, 8, and 9). The
strong AF emissions of these laminae indicate that they contain
high concentrations of organic matter (Figs. 7D–7G and 9). The
bright AF of biolm laminae in polished travertine hand samples
that had not been impregnated with epoxy indicates that these
bright AF emissions are from the travertine itself and not the
epoxy (Fig. 7G). Furthermore, CL petrography has been used
to characterize the types of individual events of both original
carbonate sedimentation and secondary diagenetic alteration
observed within the Anio Novus aqueduct travertine (Barker and
Kopp, 1991; Fouke et al., 2005; Meyers and Lohmann, 1985).
The diffuse, dark brown biolm laminae within the travertine
shrubs generally exhibit a dull to bright orange CL (Figs. 9C, 9F,
and 10). CL emissions in these types of carbonates are primar-
ily caused by Mn2+ and some trace elements, which have a high
distribution coefcient (KD) and are mobilized under reducing
aqueous conditions (Barker and Kopp, 1991). Therefore, if the
biolm laminae CL represents an original depositional signal
and not diagenetic alteration, this may indicate periods of low-
oxygen (dysoxia) within the owing aqueduct waters where
Mn2+ and rare earth elements were mobilized and concentrated
due to high water-to-travertine ratios during deposition (Banner
and Hanson, 1990a; Fouke et al., 1996; Fouke et al., 2005). Sim-
ilar original travertine shrubs have been observed in travertine
deposited within cisterns and around lead pipes of the Baths of
Caracalla in Rome and the Castellum Aquae at Porta Vesvuii in
Pompeii, which exhibit bright AF and CL in otherwise minimally
altered travertine (Hostetter et al., 2011).
The original crystalline shrubs comprising the Anio Novus
aqueduct travertine are also comparable to travertine crystalline
shrubs that grow within modern-day natural terrestrial springs,
rivers, and caves around the world (Amundson and Kelly, 1987;
Capezzuoli et al., 2014; Chafetz et al., 1991; Dong et al., 2019;
Dreybrodt et al., 1992; Fouke, 2001, 2011; Fouke et al., 2000;
Hauck et al., 2012; Pentecost, 2005; Sanders and Friedman, 1967).
The primary difference is the small 1–3 m size of the euhedral
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 19
calcites and their dendritically branching aggregates within the
Anio Novus aqueduct travertine at Roma Vecchia, which con-
tinue to grow even during biolm laminae deposition (Fig. 7).
Previous studies of travertine crystalline shrubs within natural
settings have described the biolm laminae as “micrite layers”
(Passchier et al., 2021). However, SRAF 140-nm-resolution
microscopy illustrates that the crystalline, dendritically branch-
ing travertine shrubs are syn-depositional with the biolm lami-
nae (Fig. 7). Therefore, the dark biolm laminae are not simply
micrite layers composed of randomly organized, ≤3-m-diameter
calcite crystals (Bathurst, 1975). The primary factors controlling
precipitation of these types of travertine crystalline shrub fabrics
in natural settings include (Ford and Pedley, 1996; Fouke, 2001,
2011; Fouke et al., 2000; Pentecost, 1995a, 2005): (1) water
chemistry (e.g., major and minor elemental abundance, isotopes,
pH, HCO3, pCO2, and resultant saturation state); (2) physical pro-
cesses (e.g., degassing, temperature change, steaming, boiling,
dilution, and evaporation); (3) hydrology (e.g., ow rates, ux,
and surface area); and (4) biotic activity (e.g., microbial photo-
synthesis, respiration, and biochemical effects).
Biolm Laminae
The distinctive high-frequency stratigraphy of alternating
dark brown and light beige laminae, a hallmark of aqueduct trav-
ertine, was previously used to estimate aqueduct water chemis-
try, ow conditions, and maintenance (Aicher, 1995; Bobée et
al., 2011; Brinker, 1986; Carlut, 2011; Carlut et al., 2009; Car-
rara and Persia, 2001; Coates-Stephens, 2003a, 2003c; Dubar,
2006a, 2006b; Garbrecht and Manderscheid, 1992; Gilly et al.,
1971; Gilly, 1971; Hodge, 1992; Lombardi, 2002; Schulz, 1986;
Wilson, 2004). These studies assume that the individual dark–
light laminae couplets represent annual depositional events and
therefore attempt to use these stratigraphic sequences to date the
aqueduct travertine in a manner analogous to that of tree ring,
speleothem lamination, and lake varve chronology (Baker et
al., 2008; Faraji et al., 2021; Haneca et al., 2009; Ojala et al.,
2012; Zolitschka et al., 2015). This approach requires that the
dark brown laminae represent times when organic matter (plant
material, soil particles, tannins, humic acids, other biomolecules,
and Mn2+ adsorbed onto organic matter or as extremely ne MnO
particles) was washed into the Anio Novus during periods of sea-
sonal storm events (Barceloux, 1999; Lester and Birkett, 1999;
Passchier et al., 2021). If correct, this would be consistent with
the bright green AF and bright orange CL observed in most of
the dark laminae within the Anio Novus travertine (Figs. 10 and
11). Further supporting evidence comes from Frontinus, who
described periods of increased soil turbidity in the Anio Novus
during storms despite the use of settling tanks at the aqueduct
intake. This is also seen in modern-day analyses of the Aniene
River water at Subiaco, where increased discharge during storms
correlates with an increase in suspended solids and turbidity
(Bono and Percopo, 1996).
However, this assumption that the dark biolm laminae
represent storm events is not without its uncertainties. Stud-
ies of travertine deposition in natural systems such as caves,
lakes, springs, and marine settings around the world have docu-
mented multiple environmental conditions that can form dark,
organic matter-rich laminae (Baker et al., 2008; Dabkowski
et al., 2016; Spear and Corsetti, 2013). Dark biolm laminae
in speleothems are often yellow to brown in color and exhibit
bright green AF, which has been interpreted as due to rainfall
events that washed in soil organic matter rich in humic and ful-
vic acids (Baker et al., 2008; Gascoyne, 1978; White, 1981).
However, for high-frequency speleothem laminae, as well as
varved lake sediments, the stratigraphic positions of absolute
age dates for seasonal climatic change do not consistently or
precisely align with the dark–light laminae stratigraphy (Baker
et al., 2008; Faraji et al., 2021; Haneca et al., 2009; Ojala et al.,
2012; Zolitschka et al., 2015). Therefore, other physical, chem-
ical, and biological processes may also inuence the formation
of the dark biolm laminae, which indicates that multiple lines
of evidence need to be assembled when attempting to use aque-
duct travertine biolm laminae stratigraphy for chronological
correlations and paleoclimatic reconstructions. Furthermore,
the majority of the dark biolm laminae preserved within the
Anio Novus travertine are ner (Figs. 5–7) than the 1 m aver-
age diameter of microbial cells (Fouke, 2011). Therefore, by
analogy, these extremely thin, dark biolm laminae would con-
tain cell debris and biomolecules, but not complete cells, as is
observed in the biolm laminae of calcium oxalate human kid-
ney stones (Sivaguru et al., 2020; Sivaguru et al., 2018a). As a
result, the mechanisms responsible for forming the dark bio-
lm laminae in the Anio Novus aqueduct travertine, and other
similar types of carbonate deposits, remain to be systematically
tested and determined.
The potential role of microbial inuence on Anio Novus
aqueduct travertine deposition can also be inferred via com-
parison with crystalline calcite shrubs that precipitate at tem-
peratures of less than 25 °C and a pH of 8 in the Distal Slope
Facies at Mammoth Hot Springs in Yellowstone National Park
(Figs. 12A–12B) (Fouke, 2011). Here, the dendritically branch-
ing calcite crystalline shrubs are fully covered with microbial
biolms composed of cells and biomolecules that include extra-
cellular polymeric substances (EPS; Figs. 12A–12B). Culture-
independent 16S rRNA gene sequence surveys indicate that
these biolms are composed of a moderately diverse microbial
assemblage. However, despite the fact that these hot springs are
not impacted by seasonal variations in rainfall (Fouke, 2011),
travertine deposited in the Distal Slope Facies still forms dark
and light biolm laminae (Fouke, 2011). In addition, eld exper-
imentation has demonstrated that microbial cells, biomolecules,
and EPS (Figs. 12A–12B) cause dramatic increases in travertine
precipitation rate via protein catalysis (Fouke, 2011; Kandianis
et al., 2008). The abundance of entombed, dark brown, organic
matter-rich biolm laminae within the Anio Novus travertine
implies that this type of microbial biolm inuence may also
occur during formation of the rippled travertine surfaces within
the Anio Novus aqueduct.
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20 Sivaguru et al.
Diagenetic History of the Anio Novus Aqueduct Travertine
Diagenetic Origin of the Radiaxial Calcites
The radiaxial calcites observed within the Anio Novus aque-
duct travertine are similar to crystals observed in marine limestone
and cave speleothems (Kendall, 1985; Kendall and Broughton,
1978; Kendall and Tucker, 1971, 1973). However, the complex
growth features of these radiaxial calcite crystals in marine and
cave deposits have led to controversial and often contradictory
interpretations of their formation. As a result, it is likely that radi-
axial calcites can form as original depositional features, as well as
be the product of diagenetic alteration, depending on the specic
environmental conditions of deposition and diagenesis (Kendall,
1977; Kendall, 1985; Kendall and Broughton, 1978; Kendall et al.,
1985; Tucker and Kendall, 1973). Furthermore, the Anio Novus
aqueduct’s travertine radiaxial calcite crystals are morphologi-
cally distinct from dendritic feather calcite crystals formed in ter-
restrial springs (Jones and Renaut, 2009; Turner and Jones, 2005)
as well as from fracture-lling calcite cements precipitated under
high temperature and pressure (Passchier and Trouw, 1998). In the
Figure 11. High-resolution bright eld
(BF) and merged BF with ring aperture
contrast (RAC) microscopy images of
an Anio Novus aqueduct travertine thin
section collected from the upstream 0 m
site at Roma Vecchia (sample RNRV3-
2A; Figs. 1B and 1C and 3–5) are
shown. Radiaxial calcites (RC; a light
blue-gray color under merged BF and
RAC) grow within the travertine shrubs
(TS; a mottled yellow gray color un-
der merged BF and RAC) and form
upward-radiating branching patterns
of crystal growth, which are relatively
symmetrical in both the downstream
and upstream directions. Each radiating
branch of RC crystals is also discontinu-
ous and separated by partial to complete
intervals of crystalline TS. Dark brown
biolm laminae (example tracing shown
with black arrows in image A) are thick-
er and more diffuse than laminae within
the TS, while age-equivalent laminae
within the RC are ner and more sharp-
ly dened. An example of the lateral
continuity of the dark laminae between
TS and RC is indicated with white ar-
rows. The RC crystals exhibit upward-
oriented rhombohedral crystal termi-
nations (examples are shown with red
arrows) or irregular porous textures (see
Fig. 12) as they replace TS and crosscut
the dark–light laminae stratigraphy. (A)
Middle section of Unit 1 travertine de-
posit with linguoid and sinuous ripples
shows TS and RC. (B) Same location as
in image A, shown as a merger of bright
eld (BF) with ring aperture contrast
(RAC) images. LS—lee sands.
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 21
Figure 12. High-resolution bright eld
(BF) merged with ring aperture contrast
(RAC) microscopy images of an Anio
Novus aqueduct travertine thin sec-
tion collected from the upstream 0 m
site at Roma Vecchia (sample number
RNRV3-2A; Figs. 1B and 1C and 3–5)
are shown. (A–B) Merger of BF and
RAC images, which gives the traver-
tine shrubs (TS) a mottled yellow and
gray color, while the radiaxial calcites
(RC) appear blue. Dark brown biolm
laminae (example tracing is shown with
black arrows) are thicker and more dif-
fuse within the TS, while age-equivalent
laminae within the RC are ner and
more sharply dened. (B–C) Enlarge-
ment of white boxes labeled B and C,
respectively, in image A. Dark biolm
laminae are smooth and attened in
older, stratigraphically lower (white
arrows) portions of RC crystals. Con-
versely, the younger, stratigraphically
higher crystalline terminations of these
same upward-radiating RC crystals ex-
hibit either sharp rhombohedral crystal
terminations or irregular and porous tex-
tures (red arrows). Neither of these later
stage crystalline shapes are consistent
with the smooth, dark biolm laminae
within the RC crystals, which suggests
that the RC are in the process of diage-
netically replacing the original crystal-
line TS.
present study, the hydraulic setting of the Anio Novus aqueduct
is combined with high-resolution microscopy of the travertine to
suggest that the radiaxial calcite crystals in this setting formed as
a result of diagenetic replacement of the original travertine crys-
talline shrubs and dark biolm laminae (Bathurst, 1975; Mcil-
lreath and Morrow, 1990; Tucker and Bathurst, 1990; Tucker and
Wright, 1990). These multiple lines of evidence are summarized
and evaluated in the following discussion.
An important contextual consideration is that the large,
upward-branching radiaxial calcite crystals within the travertine
(Figs. 3–5 and 9) are inconsistent with their original precipita-
tion within the high-velocity hydraulic environment of the Anio
Novus aqueduct. The evolution of the aqueous boundary layer
thickness along the wetted perimeter of the aqueduct would
have induced shear forces onto the growing upper surface of
the travertine (Niño et al., 2003). Combined with erosion during
transport of the lee sands (Fig. 7), these hydraulic factors would
have prevented the growth of upward-branching radiaxial calcite
crystals on the uppermost surfaces of the ripple-marked traver-
tine. Furthermore, the upward-growing, radiaxial calcite crystals
are symmetrical, branching in both upstream and downstream
directions (Fig. 11). As has been documented in the travertine
Proximal Slope Facies of hot-spring drainage channels, traver-
tine shrubs that extend above the overlying boundary layer grow
into and toward the direction of ow from which dissolved ions
are being delivered (Fouke, 2011). Therefore, the symmetrically
branching growth structures of the radiaxial calcite are inconsis-
tent with their growth in the high-velocity, unidirectional ow of
the aqueduct water.
Another factor is that as the upward-branching radiaxial cal-
cites aggrade within the travertine stratigraphic section, they track
upstream to downstream shifts in the position of the ripple crests
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22 Sivaguru et al.
(Figs. 3–6). The hydraulic shear force of the owing aqueduct
water would have been strongest at the ripple crests and there-
fore could even more effectively prevent the growth of branch-
ing radiaxial crystals (Raudkivi, 1963). As a result, this suggests
that the concentration of radiaxial calcites within the ripple crest
traver tine reects preferential diagenetic alteration of the ripple
crest travertine shrubs rather than original growth in this hydrau-
lically vulnerable high-shear position (Figs. 3–6). The Anio
Novus aqueduct’s travertine radiaxial calcites also consistently
exhibit upward-radiating, branching crystalline growth structures
within the travertine deposits, which are relatively symmetrical
in both the downstream and upstream directions (Fig. 11). Each
upward-radiating syntaxial branch of radiaxial calcite crystals
is itself discontinuous, being irregularly separated by partial to
complete intervals of crystalline travertine shrubs. This discon-
tinuous, syntaxial nature of individual radiaxial calcite branches
is consistent with a diagenetic origin, whereas pauses in primary
crystallization would not result in C-axis continuity after inter-
vals of crystalline shrub growth.
Dark biolm laminae (Figs. 11 and 12) are thicker and more
diffuse within the travertine shrubs, while age-equivalent laminae
within the RC are ner and more sharply dened. When con-
sidered within the stratigraphic context of each radiaxial calcite
crystalline branch, the older, dark biolm laminae at the bottom
are smooth and attened (Fig. 12). These dark biolm laminae
shapes within the radiaxial calcite are consistent with travertine
shrub surfaces observed in cross-section and on ripple-marked
bedding planes (Figs. 2F and 4D). Conversely, the uppermost
younger terminations of branches of radiaxial calcites exhibit
either: (1) euhedral, rhombohedra-shaped crystal growth termi-
nations of tens to hundreds of microns in scale; or (2) irregu-
lar, porous, and jagged anhedral crystalline textures (Fig. 12).
Therefore, neither of these crystal termination shapes and tex-
tures are consistent with the smooth and rounded dark biolm
laminae within the radiaxial calcite crystals (Figs. 11 and 12).
If the radiaxial calcites were syn-depositional with the travertine
shrubs, which are in situ crystalline growths and not the result of
micritic sedimentation or cementation (Fig. 8), the dark brown
biolm laminae would have grown over and preserved the rhom-
bohedral-shaped geometry and/or porous textures of the radiaxial
calcite crystal terminations (Figs. 8 and 9). This strongly sug-
gests that the biolm laminae formed on the growing upper sur-
face of small, rounded travertine shrubs (Figs. 2F and 4D) that
subsequently underwent fabric-preserving (mimetic) diagenetic
replacement (Figs. 8 and 9). This interpretation is consistent with
previous observations within speleothem stalagmites, where dark
biolm laminae preserve rhombohedra-shaped crystal termina-
tions within original, unaltered radiaxial columnar calcites (Ken-
dall and Broughton, 1978; Frisia, 2015).
A diagenetic replacement origin for the radiaxial calcites
within the aqueduct travertine is further suggested by the irregular
lateral gradational contacts observed between crystalline shrubs
and radiaxial calcites (Figs. 8D–8F). If the radiaxial calcites were
syn-depositional, the travertine shrubs and their small, dendriti-
cally branching euhedral calcites would exhibit abrupt contacts
abutting the lateral faces of the radiaxial calcite crystals. Instead,
the individual 1–3-m-diameter euhedral calcite crystals com-
prising the travertine shrubs exhibit a gradational lateral contact
with some calcite crystals, which further implies a replacement
process (Figs. 8D–8F). In addition, age-equivalent, individual
dark biolm laminae that are diffuse within the travertine shrubs
become ner, more sharply dened, bright AF laminae within
the large, clear radiaxial crystals (Figs. 11 and 12). The bright
orange CL character of the dark biolm laminae within the radi-
axial crystals (Figs. 9C, 9F, and 10) may further imply that they
are mimetically preserved remnants from the original travertine
shrubs. In addition, the irregular and highly porous textures of
many of the radiaxial crystals (Fig. 12) are consistent with the
dissolution and diagenetic alteration commonly observed in car-
bonate deposits (Bathurst, 1975; Mcillreath and Morrow, 1990;
Tucker and Bathurst, 1990; Tucker and Wright, 1990).
Diagenetic Analog
Comparison of 0-year-old and 100-year-old Proximal Slope
Facies travertine deposits at Mammoth Hot Springs (Fig. 13;
Fouke, 2011) reveals diagenetic radiaxial calcite replacement
alteration fabrics similar to those observed in the Anio Novus
aqueduct travertine. The Proximal Slope Facies travertine was
deposited at higher temperatures than the Anio Novus aque-
duct travertine and is composed of aragonite rather than calcite
(Fouke, 2011). However, diagenetic alteration of the Proximal
Slope Facies provides important insight into how crystalline
travertine shrubs can be recrystallized into radiaxial calcites dur-
ing meteoric diagenesis. The Proximal Slope Facies travertine at
Mammoth Hot Springs is composed of 1–30-m-long aragonite
needles organized into shrub-like, dendritically branching, crys-
tal aggregate growth structures on the spring outow drainage
channel oor (Figs. 13C and 13D; Fouke, 2001, 2011; Fouke
et al., 2000). The 0-year-old, newly deposited travertine shrubs
exhibit no CL emissions (Figs. 13C and 13D). Conversely, the
original, non-CL travertine shrubs are diagenetically replaced by
mauve to bright orange CL that radiates upward and branching,
radiaxial calcite crystals (Figs. 13E and 13F). These CL emis-
sions from radiaxial calcites within travertine preserved in a non-
burial meteoric environment at Mammoth Hot Springs, a diage-
netic environment analogous to that of the Anio Novus aqueduct
travertine, suggest that this diagenetic replacement alteration was
caused by high water-to-travertine reaction ratios with dysoxic
diagenetic freshwaters in which Mn2+ and other trace elements
were mobilized (Banner and Hanson, 1990b; Barker and Kopp,
1991; Brand and Veizer, 1980, 1981; Fouke et al., 1996; Fouke et
al., 2005; Richter et al., 2003).
Implications for Archaeological Reconstructions
Reconstructions of Human History and Paleoclimate
Several previous studies have attempted to reconstruct
the timing, velocity, chemistry, and source area of the ancient
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 23
aqueduct’s owing waters by analyzing the dark–light laminae
stratigraphy that is the hallmark of Roman aqueduct travertine
(Aicher, 1995; Brinker, 1986; Carlut, 2011; Carlut et al., 2009;
Carrara and Persia, 2001; Coates-Stephens, 2003a, 2003b,
2003c; Dubar, 2006a, 2006b; Garbrecht and Manderscheid,
1992; Gilly et al., 1971; Gilly, 1971; Hodge, 1992; Lombardi,
2002; Schulz, 1986). Collectively, these studies have proposed
that the dark–light laminae couplets reect a variety of processes,
including: (1) changes in paleoclimate, seasonal temperature, and
solar cycles; (2) variations in water velocity and turbulence, mix-
ing of waters, water chemistry, pressure, and depth; (3) chemi-
cal reaction with the surrounding aqueduct mortar and building
stones; and (4) periods of disuse and other archaeological events.
However, as described previously, uncertainties surrounding the
Figure 13. Images show crystalline
structure, microbial biolms, and diage-
netic alteration of low-temperature trav-
ertine deposited in spring outow drain-
age channels at Mammoth Hot Springs
in Yellowstone National Park. (A)
Environmental scanning electron mi-
croscope (ESEM) images of travertine
precipitated in the cool (<25 °C) drain-
age patterns to form the Distal Slope
Facies at Spring AT-1 within the Mam-
moth Hot Springs complex (described in
Fouke, 2011). Euhedral calcite crystals
form dendritically branching aggregates
(travertine shrubs, TS) that are coated
with biolms containing microbial cells
and strands of extracellular polymeric
substances (EPS; described in Fouke,
2011). (B) Enlargement of white box in
image A. (C–D) Paired brighteld (BF;
left) and cathodoluminescence (CL;
right) images taken in the same location
of a thin section of modern-day, actively
growing (0 age) Proximal Slope Facies
aragonite TS precipitated in the warmer
45–50 °C drainage outow channel of
Spring AT-1 at Mammoth Hot Springs
(modied from Fouke, 2011). (D) Note
that the original aragonite TS shown in
these images emits no CL light. (E–F)
Paired BF (left) and CL (right) images
taken in the same location of a thin sec-
tion of 100-year-old Proximal Slope Fa-
cies aragonite TS deposited at Highland
Terrace in Mammoth Hot Springs (mod-
ied from Fouke, 2011). Note that the
original CL-extinct TS exhibits mauve-
CL, early-stage diagenetic alteration
(white arrows) and bright orange CL re-
placement radiaxial calcite crystals (la-
beled RC; modied from Fouke, 2011).
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24 Sivaguru et al.
determination of the underlying mechanisms that control forma-
tion of the dark laminae (Baker et al., 2008; Faraji et al., 2021;
Haneca et al., 2009; Ojala et al., 2012; Zolitschka et al., 2015)
call into question the accuracy of many of these interpretations.
Recently, a series of benchmark studies presented standard
petrographic and high-resolution chemostratigraphic analyses of
Roman aqueduct travertine deposits in France, Turkey, and Jor-
dan (Passchier and Surmelihindi, 2019; Passchier et al., 2021;
Passchier et al., 2016a; Passchier et al., 2016b; Passchier et al.,
2013; Sürmelihindi, 2013; Sürmelihindi, 2018; Sürmelihindi et
al., 2018; Sürmelihindi and Passchier, 2013; Sürmelihindi et al.,
2013a; Sürmelihindi et al., 2019; Sürmelihindi et al., 2013b).
As is the case for the Anio Novus aqueduct, many of the walls
and ceilings in these other aqueducts distributed throughout
the ancient Roman Empire are in ruins or completely absent.
This has exposed the aqueduct travertine deposits to millennia
of freshwater percolation that may have resulted in diagenetic
alteration of their original crystalline fabrics and geochemi-
cal composition (Morse and McKenzie, 1990). Field and hand
sample photography and standard petrographic analyses (BF
and POL) of 30–35-m-thick sections were used in these studies
to suggest that the original depositional fabric of the aqueduct
travertine was composed of: (1) dark laminae of ne-grained
micrite, which appear opaque at the 3–5 m resolution of the
petrographic microscopes that were used and (2) light laminae
of dense, coarse-grained, and transparent “sparite” calcite crys-
tals elongated along their c-axes (Passchier and Surmelihindi,
2019; Passchier et al., 2021; Passchier et al., 2016a; Passchier et
al., 2016b; Passchier et al., 2020; Passchier et al., 2013; Sürme-
lihindi, 2013; Sürmelihindi, 2018; Sürmelihindi et al., 2018;
Sürmelihindi and Passchier, 2013; Sürmelihindi et al., 2013a;
Sürmelihindi et al., 2019; Sürmelihindi et al., 2013b).
Although uncertain, the “dark micrite laminae” described
in these previous studies may be similar to the travertine shrubs
observed in the Anio Novus travertine, which are composed of
micrite-sized, 1–3-m-diameter, dendritically branching cal-
cite crystals (Figs. 7A–7C). These ne crystalline shrub fabrics
would appear as dark, opaque micrite when analyzed under
BF and POL on standard petrographic microscopes within
30–35-m-thick petrographic thin sections. In addition, the light-
colored “sparite” laminae observed in other aqueduct travertine
may be comparable to the radiaxial calcite mimetic replacement
crystals observed in the Anio Novus aqueduct travertine (Figs.
3–5 and 9). Previous descriptions (e.g., Passchier et al., 2021)
include: (1) heterogeneous crystal distributions along individual
laminae, which are comparable to radiaxial calcite distributions
observed in the present study (Fig. 8C); and (2) large crystals
that cross-cut the dark–light laminae stratigraphy and commonly
radiate vertically to connect through the entire thickness of the
deposit, which are comparable to radiaxial calcite distributions
observed in the Anio Novus aqueduct travertine (Fig. 9).
These previous studies of Roman aqueduct travertine distrib-
uted throughout the Roman Empire also presented high- resolution
δ18O, δ13C, and trace element analyses conducted within “sparite”
calcite crystals (Claes et al., 2017a; Claes et al., 2017b; Claes
et al., 2015; Passchier and Surmelihindi, 2019; Passchier et al.,
2021; Passchier et al., 2016a; Passchier et al., 2016b; Passchier
et al., 2020; Passchier and Sürmelihindi, 2010; Passchier et al.,
2013; Sürmelihindi, 2013; Sürmelihindi, 2018; Sürmelihindi
and Passchier, 2013; Sürmelihindi et al., 2013a; Sürmelihindi et
al., 2019; Sürmelihindi et al., 2013b). Chemostratigraphic data
in these studies exhibit consistent trends in δ18O, δ13C, and trace
element composition, which strongly suggest they record origi-
nal seasonal paleoclimatic trends in water temperature, ow rate,
rainfall, and aquifer recharge at the time of aqueduct travertine
deposition (Passchier and Surmelihindi, 2019; Passchier et al.,
2021; Passchier et al., 2016a; Passchier et al., 2016b; Passchier
et al., 2020; Passchier et al., 2013; Sürmelihindi, 2013; Sürme-
lihindi, 2018; Sürmelihindi and Passchier, 2013; Sürmelihindi et
al., 2013a; Sürmelihindi et al., 2019; Sürmelihindi et al., 2013b).
Combined with standard petrography, this comprehensive data
set was used to reconstruct a history of maintenance, oods,
droughts, and earthquakes (Passchier et al., 2021). The dark
laminae preserved within their radiaxial “sparite” calcite crystals
(Passchier et al., 2021) may be similar to the crystalline replace-
ment fabrics observed in the Anio Novus radiaxial calcites (Figs.
8 and 9). Preservation of original chronostratigraphic trends in
mimetic replacement calcites has been identied in marine car-
bonates (Fouke et al., 1996; 2005) and would be consistent with
the mimetic replacement radiaxial calcites observed within the
Anio Novus aqueduct travertine.
Future Studies
Integration of the eld, hand sample, and high-resolution
microscopy analyses conducted for the present study, combined
with eld, petrographic, and chemostratigraphic approaches
established in previous work (Bobée et al., 2011; Hostetter et
al., 2011; Passchier and Surmelihindi, 2019; Passchier et al.,
2021; Passchier et al., 2016a; Passchier et al., 2016b; Passchier
et al., 2020; Passchier et al., 2013; Sürmelihindi, 2013; Sürme-
lihindi, 2018; Sürmelihindi et al., 2018; Sürmelihindi and Pass-
chier, 2013; Sürmelihindi et al., 2013a; Sürmelihindi et al., 2019;
Sürmelihindi et al., 2013b), will permit future studies to compre-
hensively and routinely evaluate the potential impact of diage-
netic alteration on reconstructions of paleoclimate and archaeo-
logical information from travertine deposited in ancient water
transport and storage systems around the world. This systematic
approach would include: (1) establishment of a Powers of Ten
3-D contextualization for eld, hand sample, and thin section
analyses; (2) optical, laser, electron, and X-ray microscopy anal-
yses; (3) quantitative measurement and correlation of the dark–
light laminae stratigraphic layering sequences; (4) expansion of
the aqueduct travertine chemostratigraphic data set to include
carbonate-relevant δ18O, δ13C, and 87Sr/86Sr as well as Ca, Mg, Sr,
Mn, and Fe elements with distribution coefcients (KDs) that are
both greater than and less than one (Banner and Hanson, 1990;
Brand and Veizer, 1980, 1981); (5) covariation modeling to eval-
uate and correct for potential mixing during sampling between
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Diagenetic history of travertine in the Anio Novus aqueduct of ancient Rome 25
the original travertine and diagenetic components (Langmuir et
al., 1978; Sivaguru et al., 2019a), as well as cumulative water-
rock molar ratio mass balance diagenetic water-rock interactions
among freshwater and the aqueduct travertine (Banner and Han-
son, 1990; Fouke et al., 2005); (6) evaluation of multiple other
original biotic and abiotic inuences during travertine deposition
(Fouke, 2001, 2011; Fouke et al., 2000; Kandianis et al., 2008;
Veysey et al., 2008); (7) addition of U-series isotope dating if the
samples pass petrographic and geochemical screening for dia-
genetic alteration; and (8) controlled laboratory experimentation
using microuid testbeds such as the GeoBioCell (Fouke et al.,
2022) to determine how and why the aqueduct travertine ripple
marks form as a result of in situ crystal precipitation processes
rather than downstream hydraulic transport of sedimentary grains
(Keenan-Jones et al., 2022).
CONCLUSIONS
The depositional and diagenetic history of travertine depos-
ited within the ruins of the Anio Novus aqueduct of ancient
Rome was studied by combining hydraulic reconstructions with
high-resolution optical, laser, electron, and X-ray microscopy
analyses. Samples were collected at upstream to downstream
0 m and 140 m sites along a continuous run of the Anio Novus
aqueduct channel at Roma Vecchia. The depositional history of
the aqueduct travertine included precipitation of dendritically
branching aggregates of 1–3-m-diameter euhedral calcite
crystalline shrubs with high-frequency, dark–light biolm lami-
nae, linguoid, sinuous and hummocky crystal growth ripples,
and lee sand lag deposits. The diagenetic history of the Anio
Novus aqueduct travertine included the precipitation of fabric
destructive and mimetic fabric preserving upward-branching
radiaxial calcite replacement crystals, which cross-cut the
dark–light laminae stratigraphy and crystal growth ripple bed-
forms. Future studies aimed at reconstructing human activity
and paleoclimate will be able to incorporate these approaches
to create the type of depositional and diagenetic frameworks
required for more accurate reconstructions from travertine
deposited in other ancient water conveyance and storage sys-
tems around the world.
ACKNOWLEDGMENTS
This research was completed in recognition of, and appreciation
for, the innumerable personal and professional lifetime achieve-
ments of Walter Alvarez at the University of California, Berke-
ley. People around the world have had their curiosity ignited,
and their fundamental approach to scientic inquiry redirected,
by Walter’s avant-garde research vision, engaged teaching,
and inspirational writing. Walter infused his passion for Ital-
ian language, food, art, music, and history into all of us who
were members of his Renaissance Geology research group. We
gratefully acknowledge that permission to conduct this research
was provided by the Soprintendenza per i Beni Archeologici del
Lazio (especially Dott. Zaccaria Mari) and the Soprintendenza
Speciale per i Beni Archeologici di Roma (especially Arch. Gia-
como Restante). This research was supported by the Andrew W.
Mellon Foundation through the Illinois Program for Research
in the Humanities, the Italian Government, the late William and
Janet Gale, Macquarie University, the British Academy and
British School in Rome, the Ed and Barbara Weil Fund for Uni-
versal Biomineralization at the University of Illinois Urbana-
Champaign, and the National Aeronautics and Space Adminis-
tration (NASA) Astrobiology Institute (cooperative agreement
NNA13AA91A) issued through the Science Mission Director-
ate. The support of the Chester and Helen Siess Professorship
and the M.T. Geoffrey Yeh Chair in Civil and Environmental
Engineering at the University of Illinois Urbana-Champaign is
also gratefully acknowledged. We also thank Glenn Fried for
modifying and enlarging the cathodoluminescence stage and
assisting with cathodoluminescence petrography, Charlie Ker-
ans and Jeff Trop for invaluable scientic discussions, and Julia
Waldsmith and Megan Ward for assistance in the eld and labo-
ratory. Conclusions and interpretations presented in this study
are those of the authors and do not necessarily reect those of
the funding agencies and permitting entities.
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