Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a

Abstract

The early Aptian Oceanic Anoxic Event (OAE 1a) resulted from an exceptional set of interactions between the geosphere, the biosphere, and the ocean-atmosphere system. We present new Re-Os data from two sites spanning OAE 1a in the Tethys and Pacific Oceans. The patterns of variation in the seawater Os-isotope composition from both sites are very similar, and together they constrain the timing and duration of continental weathering in relation to the large-scale volcanic activity of the Ontong Java Plateau. The dominant feature through the OAE is an interval of similar to 880 k.y. when the Os-isotope composition of the global ocean was exceptionally unradiogenic, implicating unambiguously the Ontong Java Plateau as the trigger and sustaining mechanism for OAE 1a. A relatively short-lived (similar to 100 k.y.) Os-isotope excursion to radiogenic compositions in the Tethyan record is clearly linked to an abrupt perturbation to the global carbon cycle, and is fully consistent with the Pacific record. These highly distinctive features of seawater Os in contemporaneous samples from three high-resolution sections, two of which were very remote from the Ontong Java Plateau, indicate that ocean mixing at that time was very efficient. The results suggest that OAE 1a was also related to rapid global warming and elevated rates of silicate weathering both on the continents and in the oceans.

Full text

GEOLOGY
|
July 2012
|
www.gsapubs.org 583
ABSTRACT
The early Aptian Oceanic Anoxic Event (OAE 1a) resulted from
an exceptional set of interactions between the geosphere, the biosphere,
and the ocean-atmosphere system. We present new Re-Os data from
two sites spanning OAE 1a in the Tethys and Pacifi c Oceans. The pat-
terns of variation in the seawater Os-isotope composition from both
sites are very similar, and together they constrain the timing and dura-
tion of continental weathering in relation to the large-scale volcanic
activity of the Ontong Java Plateau. The dominant feature through the
OAE is an interval of ~880 k.y. when the Os-isotope composition of the
global ocean was exceptionally unradiogenic, implicating unambigu-
ously the Ontong Java Plateau as the trigger and sustaining mechanism
for OAE 1a. A relatively short-lived (~100 k.y.) Os-isotope excursion to
radiogenic compositions in the Tethyan record is clearly linked to an
abrupt perturbation to the global carbon cycle, and is fully consistent
with the Pacifi c record. These highly distinctive features of seawater Os
in contemporaneous samples from three high-resolution sections, two
of which were very remote from the Ontong Java Plateau, indicate
that ocean mixing at that time was very effi cient. The results suggest
that OAE 1a was also related to rapid global warming and elevated
rates of silicate weathering both on the continents and in the oceans.
INTRODUCTION
During the early Aptian, widespread accumulation of sediments rich
in organic carbon occurred under oxygen-poor conditions throughout the
world’s oceans. This phenomenon, known as Oceanic Anoxic Event 1a
(OAE 1a), has its sedimentary expression in the Selli Level that crops out
in the Apennines of Marche and Umbria, Italy (Coccioni et al., 1987),
and in many other localities worldwide. It has been suggested that OAE
1a may have been triggered by the emplacement of the Ontong Java
Plateau in the central Pacifi c Ocean (e.g., Erba, 1994; Larson and Erba,
1999; Jones and Jenkyns, 2001; Méhay et al., 2009; Tejada et al., 2009).
The study by Tejada et al. (2009) found that the isotope composition of
Os throughout one of the Selli sections in Italy (Gorgo a Cerbara) was
exceptionally unradiogenic, suggesting that the Os was derived from a
mantle, young basaltic, or meteoritic source. However, proof of a causal
link between Ontong Java Plateau volcanism and the development of sea-
water anoxia has remained elusive because of the diffi culty of correlat-
ing accurately the volcanic successions of the Ontong Java Plateau with
the Tethyan sedimentary successions. Our study was therefore centered
on the Re-Os isotope analyses of suites of samples from two of the key
upper Barremian–lower Aptian sections across OAE 1a: Deep Sea Drill-
ing Project (DSDP) Site 463 (Mid-Pacifi c Mountains, which is close to the
Ontong Java Plateau volcanic center) and the distal Cismon core (southern
Alps, northern Italy) (Fig. DR1 in the GSA Data Repository1). The sedi-
mentary record of OAE 1a at Cismon is highly expanded relative to Gorgo
a Cerbara; the two sections accumulated in different basins of the Tethys
Ocean. The integrated stratigraphy and cyclochronology of the Cismon
core (Malinverno et al., 2010) provide an accurate time control, enabling
us to correlate the two sites at high temporal resolution. Details of the sec-
tions and of the analytical methods are provided in the Data Repository.
RESULTS
Re and Os abundances of samples from both sites show similar
trends (Fig. 1); abundances increase markedly during OAE 1a, reaching
1GSA Data Repository item 2012176, Figures DR1–DR9, Tables DR1–
DR5, and details of studied sections and methods, is available online at www
.geosociety.org/pubs/ft2012.htm, or on request from editing@geosociety.org or
Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
Osmium-isotope evidence for volcanism, weathering, and ocean
mixing during the early Aptian OAE 1a
Cinzia Bottini1,2, Anthony S. Cohen2, Elisabetta Erba1, Hugh C. Jenkyns3, and Angela L. Coe2
1Dipartimento di Scienze della Terra “Ardito Desio”, Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milan, Italy
2Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
3Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK
GEOLOGY, July 2012; v. 40; no. 7; p. 583–586; doi:10.1130/G33140.1; 1 fi gure; Data Repository item 2012176.
© 2012 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
25
17
20
Aptian
30
M0
Barremian
35
Lower
NC 6
NC 5
NC 7 NC 7
NC 5
Barremian
A
Nannoconid crisis
Nannoconid
decline
-30 -20
Selli Level
Radiolarian bed
MarlstoneBlack shale Ash layer Chert
Limestone
S (wt%)
N
(wt%)
δ
13
C
org (‰) 192
Os
(ppb)
Lithology:
CISMON DSDP Site 463
-1 4 0 100
010
0500
02
D2
A
B
C
E
00.5
0.1
0.7
D1
D3
0
2
C
-29 -19
-6 0 6
02
605
610
615
620
625
630
635
640
645
Aptian
Lower
mbsf
Lithology
Nannofossil zone
M0
Polarity chron
NC 6
187
Os
/
188
Os
(i)
Re
(ppb)
TOC
(wt%)
CaCO
3
(wt%)
δ
13
C
carb
‰ (PDB
)
0100
010
020
142
0.1 0.7
Selli L. equivalent
Nannoconid crisis
Nannoconid decline
A
B
E
D2
D1
D3
B
Figure 1. 187Os/ 188 Os(i) (red diamonds), 192Os (pink circles), and Re
(green squares), against stratigraphy (Erba et al., 1999, 2010; van
Breugel et al., 2007; Ando et al., 2008; Malinverno et al., 2010).
mbsf—meters below seafl oor; PDB—Peedee belemnite; TOC—total
organic carbon (shaded). Geochemical labels for both A and B are
either at the top or bottom of the fi gure. A: Deep Sea Drilling Project
(DSDP) Site 463. New δ13Ccarb (black) and δ13Corg (orange). B: Cismon
core. S (violet diamonds) and N (blue circles).

584 www.gsapubs.org
|
July 2012
|
GEOLOGY
400 ppb for Re and 4.9 ppb for Os. No relationship is found between
Re and 192Os abundances (the “common” Os component) with respect
to either sulfur or total organic carbon (TOC) content, indicating that
Re and Os are incorporated into Corg-rich sediments as a result of redox
reactions (Figs. DR2 and DR3). Regression of the Re-Os isotope data for
the Cismon samples (Fig. DR4) yields an age of 120 ± 3.4 Ma (MSWD
= 190; 187Os/188Os(i) = 0.189 ± 0.019), which is very close to the age
calculated for the Selli Level at Cismon (120.21 ± 0.04 Ma at the base;
Malinverno et al., 2010). Samples from DSDP Site 463 yield a less pre-
cise age of 136 ± 19 Ma (MSWD = 1130; 187Os/188Os(i) = 0.170 ± 0.014)
(Fig. DR5). Both ages are indistinguishable, within the ascribed analyti-
cal uncertainties, from the sedimentary ages for these sections, indicat-
ing that the Re-Os isotope system has remained closed since sediment
deposition. The calculated 187Os/188Os(i) of samples is thus primary and
refl ects the Os-isotope composition of contemporaneous seawater (see
Cohen et al., 1999).
The samples from both sites studied here reveal patterns in seawa-
ter 187Os/188Os that are similar and coeval (Fig. 1), from which we defi ne
fi ve segments, A to E. Segment A displays relatively constant 187Os/188Os
(~0.5–0.6) except for a single unradiogenic value (0.35) in the lower-
most part of the Cismon core. In segment B, a moderate decrease to
less radiogenic ratios occurs before the onset of OAE 1a. In segment
C, an abrupt and short-lived increase in 187Os/188Os to 0.52 occurred
in the lowermost part of the Selli Level. At DSDP Site 463, a single
unradiogenic value precedes the radiogenic Os excursion that reaches
a maximum 187Os/188Os of 0.53. In segment D, there is a pronounced
decrease in 187Os/188Os to an average value of ~0.16, commencing at
the most prominent part of the negative carbon-isotope excursion (-ve
CIE) and lasting through the entire Selli event with persistently unra-
diogenic 187Os/188Os. Segment D can be further divided into subintervals
on the basis of minor fl uctuations: D1 displays extremely unradiogenic
187Os/188Os (~0.15) characterizing the -ve CIE interval; in D2, 187Os/188Os
are slightly higher (~0.2) in the middle of the Selli Level; and in D3
there is a recurrence of extremely unradiogenic 187Os/188Os (~0.15) in
the upper part of the Selli Level. The fi nal segment E shows a relatively
small increase in 187Os/188Os (~0.25) at the end of OAE 1a.
DISCUSSION
Re-Os Data from OAE 1a
The Re-Os data from Cismon and DSDP Site 463 (Fig. 1; Fig. DR6)
show little variation across segment A, suggesting that the dominant fl uxes
of Os (radiogenic Os from the continents and unradiogenic Os from the
hydrothermal alteration of oceanic crust) were reasonably constant. One
exception is in the lowermost part of the Cismon section, where a rela-
tively unradiogenic value (187Os/188Os ≈ 0.35) occurs close to the level at
which the nannoconid (heavily calcifi ed nannofossil) abundance starts
to decline. The paucity of samples analyzed in this interval allows us to
speculate only very tentatively that an early pulse of Ontong Java Plateau
volcanism, resulting in an input of unradiogenic Os, may have occurred at
the start of a global biocalcifi cation crisis (Erba et al., 2010). Additionally,
Pb-isotope data from the Pacifi c Ocean support a latest Barremian age for
the beginning of Ontong Java Plateau eruptions (Kuroda et al., 2011), at
the same stratigraphic level as the nannoconid decline.
Samples from segment A at Gorgo a Cerbara (Tejada et al., 2009)
have higher 187Os/188Os(i) than samples of an equivalent age from both
Cismon and DSDP Site 463; some of the Gorgo a Cerbara samples also
contain unusually abundant Os. On a diagram of 187Os/188Os(i) against 1/
[Os] (Fig. DR7), the Gorgo a Cerbara segment A samples are located well
away from the trend defi ned by the other samples from that location and
by the samples from Site 463 and Cismon. Regression of the Re-Os data
for the Gorgo a Cerbara segment A samples yields a highly imprecise age
of 186 Ma, which is much older than their accepted geological age of
ca. 120 Ma. These samples are from thin (~1 cm), fi ssile layers set within
massive limestones and marls. The distinctive Re-Os characteristics of
these samples, their lack of correct chronological information, and their
fi eld setting all suggest that their Re-Os signatures have been perturbed.
The Os-isotope records from the two sites studied here (Fig. 1;
Fig. DR6) confi rm that the onset of OAE 1a was preceded by a decrease in
the 187Os/188Os of seawater over a period of ~200 k.y. (segment B), as pre-
viously documented at Gorgo a Cerbara (Tejada et al., 2009). This obser-
vation implies that higher fl uxes of unradiogenic Os commenced globally
before the start of the OAE, and presumably refl ects an early phase of
Ontong Java Plateau volcanism.
At Cismon, segment B is followed by a relatively short-lived
(~100 k.y.) and pronounced radiogenic Os excursion (segment C) that
correlates with the fi rst part of the -ve CIE (Fig. 1; Fig. DR6). This shift
in the 187Os/188Os(i), also observed at Gorgo a Cerbara (Tejada et al.,
2009), could represent (1) a decrease in the unradiogenic Os fl ux caused
by diminished Ontong Java Plateau volcanic activity, and/or (2) a sub-
stantial increase in the total fl ux of radiogenic Os supplied to the ocean
through continental weathering. The documented rise in global tempera-
ture (e.g., Ando et al., 2008; Erba et al., 2010) during the fi rst phase of
OAE 1a is likely to have been responsible for accelerated weathering
rates and the ensuing radiogenic Os-isotope excursion, with or without a
temporary cessation in Ontong Java Plateau volcanic activity. Assuming
as end members a value of 187Os/188Os = 0.127 (close to the present-
day mantle composition) and a contribution from crustal weathering of
187Os/188Os = 1.4 (Peucker-Ehrenbrink and Ravizza, 2000), to increase
seawater 187Os/188Os to ~0.52 would require a 31% contribution of radio-
genic Os from continental runoff, which implies an increase of ~72% in
global weathering rates compared with pre-OAE conditions (segment
B). Segment C is less well defi ned in samples from DSDP Site 463,
perhaps due to incomplete core recovery and/or relatively low resolution
of sampling, although the bio- and chemostratigraphic profi les indicate
that there are no major hiatuses in this interval (Erba, 1994; van Breugel
et al., 2007; Ando et al., 2008; Erba et al., 2010).
Segment D represents a very pronounced and relatively long-last-
ing (~880 k.y.) anomaly in the Os-isotope composition of Early Cre-
taceous seawater (Fig. 1; Fig. DR6). Maintaining this exceptional sea-
water Os-isotope composition would have required a substantial shift
in the balance of the Os fl uxes to the oceans, most likely involving a
large increase in the fl ux of unradiogenic Os. A cosmogenic origin can
be ruled out since there is no evidence of a contemporaneous impact
(Tejada et al., 2004). The input of unradiogenic Os is most likely to have
resulted from the hydrothermal alteration of juvenile Ontong Java Pla-
teau basalts, as suggested by Tejada et al. (2009). The other alternative—
a substantial fall in radiogenic fl ux—is highly unlikely in view of the
abrupt global warming and increased runoff at that time. The average
seawater 187Os/188Os of ~0.16 in segment D requires an ~95% contribu-
tion of unradiogenic Os, implying an increase in the unradiogenic Os
fl ux of ~25%–50% compared with pre-event values (segment A). This
large unradiogenic Os fl ux is compatible with its supply through the
high-temperature hydrothermal alteration of juvenile Ontong Java Pla-
teau volcanics (average 187Os/188Os(i) ≈ 0.1295; Parkinson et al., 2001)
and is in accord with the estimated age of basalts from this volcanic
province (e.g., Tejada et al., 2007).
The occurrence of persistently unradiogenic Os-isotope ratios
throughout OAE 1a (segment D) in samples from both the Pacifi c and
Tethys Oceans indicates that the global seawater Os-isotope composition
was homogeneous, implying effi cient oceanic circulation and mixing over
this interval. The high-resolution stratigraphic calibration between the two
sites demonstrates that the onset of segment D was coeval in the Pacifi c
and Tethys Oceans (Figs. DR6 and DR8), thus implying that the main
eruptive phase of the Ontong Java Plateau was of suffi cient magnitude to
dominate this aspect of seawater chemistry worldwide. Minor fl uctuations

GEOLOGY
|
July 2012
|
www.gsapubs.org 585
within segment D (D1, D2, and D3) might have been due to small variations
in volcanic intensity.
There was an increase in seawater 187Os/188Os at the onset of segment
E at Cismon, just before the end of the OAE (also recognizable at Gorgo a
Cerbara; Tejada et al., 2009), suggesting that the intensity of Ontong Java
Plateau volcanism decreased, perhaps accompanied by higher continental
weathering rates.
Integration with Biotic and Geochemical Proxy Data
The new Os-isotope records can be linked with further information
available for the two study sites (Fig. DR6). Segment B precedes the onset
of OAE 1a and encompasses the nannoconid crisis, which is thought to
have occurred as a consequence of increasing volcanogenic CO2 emis-
sions from the Ontong Java Plateau (intervals 1–2 of Erba et al., 2010).
The top of segment B corresponds to the beginning of the OAE and also
to a major volcanogenic CO2 pulse (interval 3 of Erba et al., 2010). The
most radiogenic Os-isotope composition in segment C immediately fol-
lows an interval of abrupt warming and the inferred injection of CH4 into
the ocean-atmosphere system (interval IV of Méhay et al., 2009), which
may have been instrumental in triggering this phase of intense continental
weathering. Indeed, the middle and upper part of segment C correlates
with a cooling interlude and nannofossil carbonate recovery (interval 7
of Erba et al., 2010), interpreted as a consequence of lowered CO2 due to
accelerated weathering.
At Resolution Guyot (Mid-Pacifi c Mountains), a single relatively
radiogenic Sr-isotope data point has been recorded at the level of the -ve
CIE (Jenkyns et al., 1995), which correlates with segment C in the Tethyan
sections. Ca-isotope data from the same site similarly indicate, across the
equivalent time interval, an increase in continental weathering rates (Blät-
tler et al., 2011). DSDP Site 463 is located close to Resolution Guyot, so
the Pacifi c Os-isotope record of segment C might indeed contain a record
of the intensifi ed continental weathering that is displayed so clearly in the
Tethyan sections. In this regard, we note that the similarities displayed by
the Tethyan and Pacifi c Os-isotope records contrast markedly with Pb-iso-
tope data (Kuroda et al., 2011), which demonstrate the lack of an Ontong
Java Plateau Pb-isotope signature in Tethyan records.
The onset of the maximum phase of carbonate dissolution and warm-
ing (interval 8 of Erba et al., 2010) corresponds to the rapid decrease in
seawater 187Os/188Os that culminates in the exceptionally unradiogenic
Os-isotope segment D. This interval most likely represents the onset of
the most intense phase of Ontong Java Plateau volcanism, and occurs at
precisely the same point as does evidence for progressive surface-water
acidifi cation, biocalcifi cation failure within calcareous nannoplankton,
and progressive shoaling of the calcite compensation depth (CCD) (Erba
et al., 2010). The onset of a relative decrease in magmatic activity in rela-
tion to weathering processes (D2) coincides with recovery of the nanno-
plankton population and the likely deepening of the CCD (interval 12 of
Erba et al., 2010). We suggest that a temporary decrease in volcanogenic
CO2 emissions favored a partial nannofossil recovery, possibly also pro-
moted by CO2 drawdown through increased continental weathering. The
intervals of increased Ontong Java Plateau volcanism are also consistent
with marked variations in trace-metal abundance and a sharp decrease to
relatively unradiogenic Sr-isotope ratios, both of which suggest substan-
tially increased submarine hydrothermal activity (e.g., Larson and Erba,
1999; Jones and Jenkyns, 2001; Duncan et al., 2007).
Comparison with Other OAEs and Hyperthermals
Comparison of the seawater Os-isotope record of OAE 1a with
data for other Mesozoic OAEs shows some striking similarities and also
important differences (Fig. DR9). For OAE 1a, there is evidence that the
onset of anoxia and the negative δ13C anomaly were preceded by a signifi -
cant input of unradiogenic Os (Tejada et al., 2009; this study). Similarly,
unradiogenic 187Os/188Os (Turgeon and Creaser, 2008) predate the latest
Cenomanian OAE 2 (94–93.6 Ma; Ogg et al., 2008) and the positive δ13C
anomaly. These observations are fully consistent with volcanism acting
as a trigger for global anoxia. The Os-isotope data indicate major erup-
tive phases lasting for almost the entire duration of these events, namely
~880 k.y. for OAE 1a (this study) and ~550 k.y. for OAE 2 (Turgeon
and Creaser, 2008), thereby maintaining anoxic conditions over unusu-
ally large areas of the oceans for a prolonged period. However, seawater
187Os/188Os indicate that during OAE 1a, Ontong Java Plateau volcanism
was intense right up to the end of OAE 1a; conversely, the published data
suggest a progressive decline in volcanic activity during OAE 2 after the
fi rst ~190 k.y. (Turgeon and Creaser, 2008).
Accelerated continental weathering and increased runoff, implicated
as the cause of radiogenic Os-isotope excursions, have been documented
not only for OAE 1a (Tejada et al., 2009; this study) but also for the
Toarcian OAE (T-OAE, ca. 183 Ma) (Cohen et al., 2004, 2007) and the
Paleocene–Eocene Thermal Maximum (PETM, ca. 55.5 Ma) (Ravizza et
al., 2001). In all three cases, the radiogenic Os-isotope excursions cor-
respond to abrupt global negative shifts in δ13C that were very likely to
have been a consequence of CH4 and/or CO2 release into the ocean-atmo-
sphere system, and consequent global warming. For OAE 2, no radio-
genic Os-isotope spike has yet been detected; however, Sr- and Ca-isotope
data (Frijia and Parente, 2008; Blättler et al., 2011) provide evidence for
an increase in global weathering at the onset of OAE 2. The duration of
the radiogenic Os-isotope interval is ~100 k.y. for both OAE 1a and the
PETM (Zachos et al., 2005), while the T-OAE is likely to have lasted
between 168 and 324 k.y., depending on interpretation of the cyclostratig-
raphy (Kemp et al., 2011).
It is noteworthy that the large-scale volcanic events that were associ-
ated with the T-OAE (the Karoo-Ferrar igneous province) and the PETM
(the North Atlantic igneous province) were predominantly continental and
subaerial. The introduction of metals and nutrients to the oceans would
thus have required the prior weathering of large volumes of subaerially
erupted basalt (e.g., Cohen et al., 2004). In contrast, the emplacement of
the Ontong Java Plateau was predominantly submarine (Tarduno et al.,
1991; Kuroda et al., 2011), and thus the direct introduction of many met-
als, including unradiogenic Os and nutrients, would have been effectively
instantaneous. Despite the differences in volcanic style and the very dis-
tinct background conditions of climate and paleogeography, many of the
major responses of the Earth system during OAEs and the PETM were
remarkably similar.
CONCLUSIONS
The new Os-isotope data demonstrate unequivocally that the effects
of submarine Ontong Java Plateau volcanism were global and that vol-
canism was directly implicated in the development of OAE 1a. These
observations also imply effi cient oceanic circulation and mixing during
OAE 1a. Correlation of the chemostratigraphic data of this study with
published cyclostratigraphy for OAE 1a enables us to constrain the tim-
ing and duration of the major phases of Ontong Java Plateau emplace-
ment and to document its interaction with both the marine and terrestrial
realms. These observations demonstrate that high-resolution records of
past global warming can provide valuable information about the behavior
of the Earth system during and after the large-scale release of carbon into
the ocean-atmosphere system.
ACKNOWLEDGMENTS
We thank Marc Davies for his unstinting and expert help with all aspects
of sample preparation and analyses, John Watson for LECO elemental analyses
(The Open University, UK), and Normal Charnley and Peter Ditchfi eld for under-
taking C-isotope analyses (Oxford). We also thank D. Selby and an anonymous
reviewer for their helpful comments on the manuscript. Bottini and Erba were
funded through grant MIUR-PRIN-2007-2007W9B2WE 001. Bottini was sup-
ported by a Cariplo Foundation grant. Analytical work at The Open University
was supported by funds from NERC and The Open University. Samples from

586 www.gsapubs.org
|
July 2012
|
GEOLOGY
Deep Sea Drilling Project Site 463 were supplied by the Integrated Ocean Drill-
ing Program.
REFERENCES CITED
Ando, A., Kaiho, K., Kawahata, H., and Kakegawa, T., 2008, Timing and magni-
tude of early Aptian extreme warming: Unraveling primary δ18O variation in
indurated pelagic carbonates at Deep Sea Drilling Project Site 463, central
Pacifi c Ocean: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 260,
p. 463–476, doi:10.1016/j.palaeo.2007.12.007.
Blättler, C.L., Jenkyns, H.C., Reynard, L.M., and Henderson, G.M., 2011, Sig-
nifi cant increases in global weathering during Oceanic Anoxic Events 1a
and 2 indicated by calcium isotopes: Earth and Planetary Science Letters,
v. 309, p. 77–88, doi:10.1016/j.epsl.2011.06.029.
Coccioni, R., Nesci, O., Tramontana, M., Wezel, C.F., and Moretti, E., 1987,
Descrizione di un livello-guida “Radiolaritico-Bituminoso-Ittiolitico” alla
base delle Marne a Fucoidi nell’Appennino Umbro-Marchigiano: Bollettino
della Società Geologica Italiana, v. 106, p. 183–192.
Cohen, A.S., Coe, A.L., Bartlett, J.M., and Hawkesworth, C.J., 1999, Precise
Re-Os ages of organic-rich mudrocks and the Os isotope composition of
Jurassic seawater: Earth and Planetary Science Letters, v. 167, p. 159–173,
doi:10.1016/S0012-821X(99)00026-6.
Cohen, A.S., Coe, A.L., Harding, S.M., and Schwark, L., 2004, Osmium isotope
evidence for the regulation of atmospheric CO2 by continental weathering:
Geology, v. 32, p. 157–160, doi:10.1130/G20158.1.
Cohen, A.S., Coe, A.L., and Kemp, D.B., 2007, The Late Paleocene–Early Eo-
cene and Toarcian (Early Jurassic) carbon isotope excursions: A comparison
of their time scales, associated environmental changes, causes and conse-
quences: The Geological Society of London Journal, v. 164, p. 1093–1108,
doi:10.1144/0016-76492006-123.
Duncan, R.A., Tiraboschi, D., Erba, E., Walczak, P.S., and Clarke, L.J., 2007,
The Cretaceous OAE1a-submarine plateau link: Additional geochemical
evidence from marine sedimentary sections: Eos (Transactions, American
Geophysical Union), v. 89, abstract T13A-1125.
Erba, E., 1994, Nannofossils and superplumes: The early Aptian “nannoconid
crisis”: Paleoceanography, v. 9, p. 483–501, doi:10.1029/94PA00258.
Erba, E., Channell, J.E.T., Claps, M., Jones, C., Larson, R.L., Opdyke, B., Pre-
moli Silva, I., Riva, A., Salvini, G., and Torricelli, S., 1999, Integrated
stratigraphy of the Cismon APTICORE (southern Alps, Italy): A reference
section for the Barremian–Aptian interval at low latitudes: Journal of Fora-
miniferal Research, v. 29, p. 371–391.
Erba, E., Bottini, C., Weissert, J.H., and Keller, C.E., 2010, Calcareous nanno-
plankton response to surface-water acidifi cation around Oceanic Anoxic
Event 1a: Science, v. 329, p. 428–432, doi:10.1126/science.1188886.
Frijia, G., and Parente, M., 2008, Strontium isotope stratigraphy in the upper
Cenomanian shallow-water carbonates of the southern Apennines: Short-
term perturbations of marine 87Sr/86Sr during the oceanic anoxic event 2:
Palaeogeography, Palaeoclimatology, Palaeoecology, v. 261, p. 15–29,
doi:10.1016/j.palaeo.2008.01.003.
Jenkyns, H.C., Paull, C.K., Cummins, D.I., and Fullagar, P.D., 1995, Strontium-
isotope stratigraphy of Lower Cretaceous atoll carbonates in the mid-Pacifi c
Mountains, in Winterer, E.L., et al., eds., Proceedings of the Ocean Drill-
ing Program, Scientifi c Results, Volume 143: College Station, Texas, Ocean
Drilling Program, p. 89–97.
Jones, C.E., and Jenkyns, H.C., 2001, Seawater strontium isotopes, oceanic an-
oxic events, and seafl oor hydrothermal activity in the Jurassic and Creta-
ceous: American Journal of Science, v. 301, p. 112–149, doi:10.2475/ajs
.301.2.112.
Kemp, D.B., Coe, A.L., Cohen, A.S., and Weedon, G.P., 2011, Astronomical forc-
ing and chronology of the early Toarcian (Early Jurassic) oceanic anoxic
event in Yorkshire, UK: Paleoceanography, v. 26, PA4210, doi:10.1029
/2011PA002122.
Kuroda, J., Tanimizu, M., Hori, R.S., Suzuki, K., Ogawa, N.O., Tejada, M.L.G.,
Coffi n, M.F., Coccioni, R., Erba, E., and Ohkouchi, N., 2011, Lead isotopic
record of Barremian–Aptian marine sediments: Implications for large igne-
ous provinces and the Aptian climatic crisis: Earth and Planetary Science
Letters, v. 307, p. 126–134, doi:10.1016/j.epsl.2011.04.021.
Larson, R.L., and Erba, E., 1999, Onset of the mid-Cretaceous greenhouse
in the Barremian–Aptian: Igneous events and the biological, sedimen-
tary and geochemical responses: Paleoceanography, v. 14, p. 663–678,
doi:10.1029/1999PA900040.
Malinverno, A., Erba, E., and Herbert, T.D., 2010, Orbital tuning as an inverse
problem: Chronology of the early Aptian oceanic anoxic event 1a (Selli
Level) in the Cismon APTICORE: Paleoceanography, v. 25, PA2203,
doi:10.1029/2009PA001769.
Méhay, S., Keller, C.E., Bernasconi, S.M., Weissert, H., Erba, E., Bottini, C., and
Hochuli, P.A., 2009, A volcanic CO2 pulse triggered the Cretaceous Oce-
anic Anoxic Event 1a and a biocalcifi cation crisis: Geology, v. 37, p. 819–
822, doi:10.1130/G30100A.1.
Ogg, J.G., CGTS, and Gradstein, F.M., 2008, The Concise Geologic Time Scale:
Cambridge, UK, Cambridge University Press.
Parkinson, I.J., and Schaefer, B.F., and ODP Leg 192 Shipboard Scientifi c Party,
2001, A lower mantle origin for the world’s biggest LIP? A high-precision
Os isotope isochron from Ontong Java Plateau basalts drilled on ODP Leg
192: Eos (Transactions, American Geophysical Union), v. 82, p. F47.
Peucker-Ehrenbrink, B., and Ravizza, G., 2000, The marine osmium isotope re-
cord: Terra Nova, v. 12, p. 205–219, doi:10.1046/j.1365-3121.2000.00295.x.
Ravizza, G., Norris, R.N., Blusztajn, J., and Aubry, M.P., 2001, An osmium iso-
tope excursion associated with the Late Paleocene thermal maximum: Evi-
dence of intensifi ed chemical weathering: Paleoceanography, v. 16, p. 155–
163, doi:10.1029/2000PA000541.
Tarduno, J.A., Sliter, W.V., Kroenke, L., Leckie, M., Mayer, H., Mahoney, J.J.,
Musgrave, R., Storey, M., and Winterer, E.L., 1991, Rapid formation of
Ontong Java Plateau by Aptian mantle plume volcanism: Science, v. 254,
p. 399–403, doi:10.1126/science.254.5030.399.
Tejada, M.L.G., Mahoney, J.J., Castillo, P.R., Ingle, S.P., Sheth, H.C., and Weis,
D., 2004, Pin-pricking the elephant: Evidence on the origin of the On-
tong Java Plateau from Pb-Sr-Hf-Nd isotopic characteristics of ODP Leg
192 basalts, in Fitton, G., et al., eds., Origin and evolution of the Ontong
Java Plateau: The Geological Society of London Special Publication 229,
p. 133–150.
Tejada, M.L.G., Suzuki, K., Hanyu, T., Mahoney, J.J., Tatsumi, Y., and Nakai,
S., 2007, Lower crustal material in the source of the Ontong Java Plateau?:
Geochimica et Cosmochimica Acta, v. 71, p. A1011.
Tejada, M.L.G., Katsuhiko, S., Kuroda, J., Coccioni, R., Mahoney, J.J., Ohkou-
chi, N., Sakamoto, T., and Tatsumi, Y., 2009, Ontong Java Plateau erup-
tion as a trigger for the early Aptian oceanic anoxic event: Geology, v. 37,
p. 855–858, doi:10.1130/G25763A.1.
Turgeon, S.C., and Creaser, R.A., 2008, Cretaceous oceanic anoxic event 2
triggered by a massive magmatic episode: Nature, v. 454, p. 323–326,
doi:10.1038/nature07076.
van Breugel, Y., Schouten, S., Tsikos, H., Erba, E., Price, G.D., and Sinninghe
Damsté, J.S., 2007, Synchronous negative carbon isotope shifts in marine
and terrestrial biomarkers at the onset of the early Aptian oceanic anoxic
event 1a: Evidence for the release of 13C-depleted carbon into the atmo-
sphere: Paleoceanography, v. 22, PA1210, doi:10.1029/2006PA001341.
Zachos, J.C., Röhl, U., Schellenberg, S.A., Sluijs, A., Hodell, D.A., Kelly,
D.C., Thomas, E., Nicolo, M., Raffi , I., Lourens, L.J., McCarren, H., and
Kroon, D., 2005, Rapid acidifi cation of the ocean during the Paleocene–
Eocene thermal maximum: Science, v. 308, p. 1611–1615, doi:10.1126
/science.1109004.
Manuscript received 20 December 2011
Manuscript accepted 27 January 2012
Printed in USA