Dynamic and thermodynamic influences on precipitation in Northeast Mexico on orbital to millennial timescales

Abstract

The timing and mechanisms of past hydroclimate change in northeast Mexico are poorly constrained, limiting our ability to evaluate climate model performance. To address this, we present a multiproxy speleothem record of past hydroclimate variability spanning 62.5 to 5.1 ka from Tamaulipas, Mexico. Here we show a strong influence of Atlantic and Pacific sea surface temperatures on orbital and millennial scale precipitation changes in the region. Multiple proxies show no clear response to insolation forcing, but strong evidence for dry conditions during Heinrich Stadials. While these trends are consistent with other records from across Mesoamerica and the Caribbean, the relative importance of thermodynamic and dynamic controls in driving this response is debated. An isotope-enabled climate model shows that cool Atlantic SSTs and stronger easterlies drive a strong inter-basin sea surface temperature gradient and a southward shift in moisture convergence, causing drying in this region.

Full text

Article https://doi.org/10.1038/s41467-023-37700-9
Dynamic and thermodynamic influences on
precipitation in Northeast Mexico on orbital
to millennial timescales
Kevin T. Wright
1
, Kathleen R. Johnson
1
, Gabriela Serrato Marks
2
,
David McGee
2
, Tripti Bhattacharya
3
, Gregory R. Goldsmith
4
,
Clay R. Tabor
5
, Jean-Louis Lacaille-Muzquiz
6
, Gianna Lum
1
&
Laura Beramendi-Orosco
7
The timing and mechanisms of past hydroclimate change in northeast Mexico
are poorly constrained, limiting our ability to evaluate climate model perfor-
mance. To address this, we present a multiproxy speleothem record of past
hydroclimate variability spanning 62.5 to 5.1 ka from Tamaulipas, Mexico. Here
we show a strong influence of Atlantic and Pacific sea surface temperatures on
orbital and millennial scale precipitation changes in the region. Multiple
proxies show no clear response to insolation forcing, but strong evidence for
dry conditions during Heinrich Stadials. While these trends are consistent with
other records from across Mesoamerica and the Caribbean, the relative
importance of thermodynamic and dynamic controls in driving this response
is debated. An isotope-enabled climate model shows that cool Atlantic SSTs
and stronger easterlies drive a strong inter-basin sea surface temperature
gradient and a southward shift in moisture convergence, causing drying in this
region.
A majority of climate models project that Northern Mexico will
become drier in the future, but the spatial distribution and magni-
tude of drying is poorly constrained at present due to a lack of model
agreement especially at the local scale1,2. Improving hydroclimate
projections for Northern Mexico is critical given the substantial
social, economic, and ecological impacts that shifts in mean pre-
cipitation or precipitation extremes can have in the region. For
instance, severe droughts in the past have led to agriculture
disruptions3, national food shortages, and have been linkedto surges
in international immigration4. Records of past hydroclimate can
provide critical constraints on the dynamical drivers of regional
precipitation variability, and contribute to improved climate
projections5, yet few records exist in Northern Mexico. Specifically,
paleoclimate records can contribute to evaluating and improving
climate models used for projecting future hydroclimate, by helping
to: (1) Constrain the magnitude and timing of precipitation change in
response to external forcings and internal ocean-atmosphere
variability6, (2) Evaluate the spatial pattern of regional precipitation
changes in models7, and (3) Provide robust data for proxy-model
comparison studies, which may help reveal model biases8. Spe-
leothems are ideally suited archives of past hydroclimate due to their
precise U-Th based age models and the multiple hydrologically sen-
sitive proxies they contain.Despite the prevalence of limestone karst
landscapes in Northeast (NE) Mexico, though, there has only been
one published speleothem record spanning the last millennium from
the region9.
Received: 10 June 2021
Accepted: 28 March 2023
Check for updates
1
Dept. of Earth System Science, University of California, Irvine, 3200 Croul Hall, Irvine, CA, USA.
2
Department of Earth, Atmospheric and Planetary Sciences,
Massachusetts Institute of Technology, Cambridge, MA, USA.
3
Department of Earth Sciences, Syracuse University, Syracuse, NY, USA.
4
Schmid College of
Science and Technology, Chapman University, Orange, CA, USA.
5
Department of Geosciences, University of Connecticut, Storrs, CT, USA.
6
Independent
researcher, Ciudad Mante, Tamaulipas, Mexico.
7
Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Ciudad de,
México, México. e-mail: ktwright@uci.edu;kathleen.johnson@uci.edu
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The modern climatology of NE Mexico (Fig. 1) is dominated by the
Caribbean Low-Level Jet (CLLJ), which transports moisture from the
Atlantic Ocean and Caribbean Sea to the Atlantic slope of Mexico and
Central America during boreal summer10,11. The strengthening of the
summer CLLJ, defined as an increase in low-level easterly wind velocity
off the coast of northern South America, is driven by increased solar
heating and a northward migration of the Intertropical Convergence
Zone (ITCZ)12. The CLLJ, however, also strengthens in February, driven
by an intensified meridional pressure gradient linked to heating over
South America. It is important to note that only the CLLJ maximum in
Boreal summer is associated with increased moisture flux and pre-
cipitation over Mexico. However, transient weather events like tropical
cyclones can alsobring heavy rainfall to the region in late summer and
early autumn13.
On orbital timescales, insolation variations dominated by pre-
cessionhave been proposed to impact regional hydroclimate in similar
ways as the seasonal ITCZ migration, with strengthening of the CLLJ
and precipitation increases occurring during Northern Hemisphere
summer insolation (NHSI) maxima when the ITCZ migrates north.
Available proxy records from NE Mexico show unclear evidence for a
strong insolation control on regional hydroclimate, though. For
instance, while a sediment record from the El Potosi Basin in NE Mexico
demonstrates a strong positive correlation of runoff to NHSI14,other
studies from the region have found a limited role for NHSI and sug-
gested that autumn or spring insolation may be more important dri-
vers of hydroclimate15,16. In contrast, the role of insolation in NW
Mexicoand Southern Mexico is much better understood, withmultiple
records demonstratinga strong positive correlation between NHSI and
precipitation via alteration of the North American Monsoon17–21 and
shifts in the ITCZ22.
On millennial timescales, precipitation variability in NE Mexico
has also been linked to the strength of the CLLJ. For instance, Roy
et al.14 found decreased Ti concentration in lake sediments from El
Potosi Basin during Heinrich Stadial (HS) 1, which were interpreted as
reflecting reduced precipitation caused by a weakening of the CLLJ as
the ITCZ shifted south. However, this interpretation may be incon-
sistent with modern dynamics of the CLLJ, which actually strengthens
during Boreal winter (February) when the ITCZ migrates south23,24.
Using the seasonal ITCZ migration as an analog, it is possible that the
CLLJ could actually strengthen during HS events, suggesting some
other factor, such as sea surface temperatures (SSTs), may play a more
important role indriving millennial-scale hydroclimate variability in NE
Mexico.
While previous records have not shown a strong SST control on
mean precipitation in NE Mexico on orbital or millennial timescales,
several previous paleoclimate reconstructions and modeling studies
have linked hydroclimate variations across Mexico, Central America
and the Caribbean to changes in SSTs. Although NE Mexico is outside
the nuclear Mesoamericanregion, due to thecultural and climatic links
of NE Mexico to Southern Mexico and Central America, we will here-
after refer to this entire region as Mesoamerica. Tree ring and climate
modeling studies have shown that both Pacific and Atlantic SSTs exert
a strong precipitation control across Mesoamerica on interannual to
multidecadal timescales25–29. Over the Common Era (last 2000 years)
changes in Atlantic SSTs have been associated with a strong, out-of-
phase, dipole precipitation pattern between northern and southern
Mesoamerica (See Fig. 5from28). However, a recent speleothem study
has suggested the dipole precipitation pattern is biased towards winter
precipitation, and precipitation on annualtimescales, and longer, may
respond more in-phase throughout the region9. Unfortunately, the
impact of SST variations on Mesoamerican hydroclimate patterns on
millennial and orbital timescales is poorly constrained due to the
paucity of records. For instance, while records from southern Mesoa-
merica consistently show drying during HS events22,29, lake sediment
records from northern Mexico demonstrate wet, dry and neutral
responses30–32. Although the inconsistency of the northern Mexico
records may simply be driven by uncertainties in the age models30,or
the influence of tropical Pacificcyclones
32, the sparse paleoclimate
record from NE Mexico hinders our ability to assess the spatial pattern
of precipitation response to SST changes on orbital and millennial
timescales.
In addition to the spatial response of precipitation to SST varia-
bility, there is some evidence to suggest SSTs may impact extreme
precipitation in NE Mexico. For instance, increased clay mineral con-
centration (Al+Si+K + Fe/Ca), interpreted to reflect increased water-
shed erosion from high intensity rainfall, in lake sediments from the
Cieneguilla Basin and the Sandia Basin in NE Mexico have linked per-
iods of increased tropical cyclones to warm Gulf of Mexico SSTs during
Fig. 1 | Summer (JJAS) climatology and nearby paleoclimate records. Map of
regional precipitation and magnitude of low-level (850 mb) winds using PERSIANN
precipitation data106 and winds from MPI-ESM-Historical107. Nearby records include
(1) an oceansediment core fromthe Gulf of Mexico59 and the (2) Florida Straight49,
speleothem records from (3) Cuba46, (4) Southern Mexico22 and (8) Costa Rica108,
additional ocean sediment cores from the (6) Caribbean Sea58, and (7) Cariaco
Basin74, and a lake sediment core (5) from Guatemala50.
Article https://doi.org/10.1038/s41467-023-37700-9
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the mid-Holocene and Bølling-Allerød15,30. However, notably, these
shifts in precipitation extremes were not clearly associated with shifts
in mean precipitation and, overall, the correlation between GOM SSTs
and NE Mexico precipitation on orbital and millennial timescales has
been shown to be weak or inconsistent14. Given these discrepancies,
additional interglacial-glacial records of hydroclimate variability are
needed to further explicate the role of SSTs on regional precipitation
change.
Specifically, the relative importance of CLLJ strength and SSTs in
driving regional hydroclimate across Mesoamerica remains an open
question. Robust paleoclimate records are needed to help resolve this
issue. To this end, we present a new decadal-resolution, multi-proxy
(δ18O, δ13C, Mg/Ca) speleothem record from Tamaulipas, Mexico that
spans 62.5 to 5.1 ka. Our results show strong hydrologic responses to
key millennial-scale events including the Younger Dryas and Heinrich
Stadials 1, 3–6, and a muted response to NHSI. Furthermore, we utilize
results of a freshwater-forcing experiment conducted with an isotope-
enabled climate model to investigate the importance of dynamic and
thermodynamic controls on NE Mexico precipitation.
Results and discussion
Multiproxy reconstruction of hydroclimate variability in NE
Mexico
We present a ~57,000 year record of hydroclimate utilizing oxygen
isotopes (δ18O), carbon isotopes (δ13C), and trace elements (Mg/Ca)
from a 78 cm-long candle-shaped stalagmite, CB2 (Fig. 2,SeeSINote1).
CB2 was collected from Cueva Bonita (23°N, 99°W; 1071 m above sea
level), located in the highlands of the Sierra Madre Oriental in the
northeastern Mexico state of Tamaulipas (Fig. 1;SeeSINote2).The
climate of this region is characterized by cool-dry winters and warm-
wet summers13 (Fig. S1), with a precipitation maximum in summer
(July) driven by warm North Atlantic SSTs and anintensification of the
Caribbean Low-Level Jet13 (Fig. 1). The stable isotope and trace element
proxy data are tied to a U-series age-depth model, constrained by 33
230Th−234U ages and the mean of 2000 Monte-Carlo simulations using
the age-modeling software COPRA33. The age model indicates the
sample formed continuously from 62.5 to 5.1 ka with an average
growth rate of ~14 μm/yr, and an average temporal resolution of
~36 years (Fig. 2). This represents the highest resolution, continuous
paleoclimate proxy record in Mexico over this time-period.
Previous work in Southern Mexico has consistently interpreted
the oxygen isotope signature of precipitation (δ18O
p
)asreflective of
precipitation amount22,34,35,36, with greater amounts of rainfall asso-
ciated with more negative δ18O
p
values. Risi et al.37 argues the amount
effect dominates in the tropics due to high rainfall rates, which limits
isotopic exchange with near-surface moisture. Furthermore, more
recent analysis in the nearby mid-latitudes has interpreted δ18O
p
to
reflect shifting moisture source, temperature, relative proportions of
stratiform vs convective precipitation, seasonality, and shifts in thun-
derstorm size and duration38–40. To improve our understanding of
modern precipitation isotope systematics at our site, we have used an
array of modeling and observational data. We analyzed moisture
bearing air trajectories over a 15-year period (See SI Note 6) which
demonstratethatmoistureisconsistentlysourcedfromtheGulfof
Mexico and Caribbean Sea. An observational record of precipitation
δ18O from approximately 1 km from the cave, established aspart of this
study, suggests the δ18O of monthly precipitation is strongly depen-
dent on precipitation amount (p<0.01; r2=0.88; slope=−2‰/
100 mm) (Fig. S2). This correlation is further supported by isotope-
enabled GCM simulations of precipitation spanning the last 40 years,
which suggest lower δ18O
p
values primarily reflect an increase in
regional precipitation amount (Fig. S2). Given the stable cave envir-
onmental conditions in CuevaBonita (Fig. S3), theminimal influence of
cave variability (temperature, evaporation, or degassing) on spe-
leothem δ18O as demonstrated by a simple proxy system model
(Fig. S4), and the fact that δ18O values of calcite precipitated on glass
plates from Cueva Bonita areclose to oxygenisotopic equilibriumwith
drip waters, suggests that speleothems from this cave preserve varia-
tions in the δ18O values of ancient drip water and precipitation (See SI
Note 8). We therefore interpret CB2 speleothem δ18Oasreflective of
regional precipitation amount in NE Mexico.
While δ18O is often a proxy for large scale atmospheric processes,
the controls of δ13C are more localized and record changes in overlying
vegetation (amount and type), soil respiration, temperature, and CO
2
degassing within caves and associated with prior calcite precipitation
(PCP) in the epikarst41,42. Despite these complex controls, speleothem
Fig. 2 | Stalagmite CB2, age-depth model and Mg/Ca, δ18Oandδ13C results.
aResults of 1578 stable isotope and 789 trace element measurements. δ18Oistop
and blue, δ13C is central and in orange, Mg/Ca is in bottom and green. Dates with
associated uncertaintiesare below Mg/Ca. Heinrich Stadialshighlightedin light red.
bCB2 Age-Depth Model constructed using 2000 Monte-Carlo simulations via the
age-depth modeling software COPRA and 33 U-Th ages. Uncertainty in age-depth
model indicated by gray shading, uncertainty in U-Th ages indicated by red error
bars. cSample CB2 after being cut and polished.
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δ13Cvalueshavebeenincreasinglyshowntoreflect local water balance,
with PCP and soil/vegetation changes all leading to higher δ13Cvalues
during drier periods9,41–43. We suggestthat a major driver of δ13CinCB2
is PCP, which occurs when there is reduced local water balance and is
the result of enhanced CO
2
degassing and calcite precipitation in the
epikarst41. To further constrain the mechanisms of CB2 δ13C variability,
we conducted Mg/Ca analyses, which can also reflect PCP. In addition
to the preferential loss of 12C from the drip water dissolved inorganic
carbon pool during CO
2
degassing, PCP leads to the preferential
uptake of Ca2+ leaving the remaining drip waters, and the speleothem,
enriched in trace elements (Mg2+
). CB2 δ13C and Mg/Ca ratios exhibit
similar variability throughout the late-Pleistocene and weakly correlate
(autocorrelation corrected r=0.39, p< 0.01) throughout this time
period, suggesting PCP influences both Mg/Ca ratios and δ13C. While
we cannot rule out the additional influence of other factors, such as
soil/vegetation changes, on speleothem δ13C or temperature changes
on Mg/Ca, our multi-proxy approach allows for a more robust inter-
pretation of our proxy record.
The CB2 δ18O record is remarkably smooth over the glacial period,
is punctuated by a several prominent millennial scale variations during
the glacial and deglacial periods (HS1, Bølling-Allerød, Younger Dryas),
and thereis a clear ~1‰decreasefrom the Last Glacial Maximum (LGM;
~21 ka) to the Holocene (after correcting for global ice volume). In
contrast to speleothem records from Asia and South America44,45,the
CB2 δ18O record shows no clear precessional signal on orbital time-
scales. While increased precipitation in the early-Holocene has been
recorded elsewhere in Mesoamerica22,46, the effects of changing
glacial-interglacial cave temperatures could easily explain the 1‰
glacial-interglacial shift observed in our δ18O record (See SI Note 10).
The lack of orbital variability provides a relatively stable and un-
varying background, which large amplitude millennial scale variability
is superimposed upon. The δ18O time series consistently exhibits large
positive excursions of ~2‰during Heinrich Stadials, indicating shifts
towards drier conditions. Speleothem δ18Ovalues(n=1578) range
from −1.29‰to −6.30‰(VPDB) with the most 18O-enriched samples
occurring during Heinrich Stadial 1 (HS 1) at ~16.8 ka (Fig. 2). However,
excursions toward higher δ18O values are also noted during 50–47 ka,
43–42 ka, 31–28 ka, 18–15 ka, and 12–10 ka, corresponding to HS 5, 4, 3,
1 and the Younger Dryas (YD), respectively (Fig. 2). While the timing of
HS 4 in our record is slightly older (~42–40) than the expected age
(~40–38 ka)47, the offset is possibly attributed to a relatively large
uncertainty in our age model around this time (Fig. 2, Fig. S5, Table S1).
The CB2 δ13C data co-varies with δ18O(r=0.53, p<0.01) and is
similarly dominated by large amplitude millennial variations during
the glacial, and a ~3‰decrease across the deglaciation (Fig. 2). A lake
sediment record from the region suggests the decrease in δ13Ccould
reflect a shift from C
4
to C
3
dominant vegetation15, but this shift could
also potentially reflect some combination of decreased PCP, increased
soil respiration or vegetation intensity, and/or decreased water-rock
interaction during the Holocene42,48. Millennial-scale shifts in δ18Oare
reproduced in the δ13C record, consistent with decreased local water
balance during the Younger Dryas and Heinrich Stadials. Increases in
δ13C were as large as 3.94‰during HS 5. However, not all changes in
δ13C were as dramatic, such as during HS 3 the shift in δ13C was as subtle
as 1.04‰. We suggest the varying responses recorded by CB2 proxies
may reflect real differences between individual Heinrich Stadials49,
though some influence of complex proxy controls may also play a role.
The CB2 Mg/Ca values exhibit similar variations on millennial
timescales as the stable isotopes, particularly to δ13C. Mg/Ca values
increase above average glacial values (27 mmol/mol) to 37 mmol/mol
during the YD, 68 mmol/mol during HS1, 34 mmol/mol during HS3,
37 mmol/mol during HS4, 35 mmol//mol during HS5, and to 33mmol/
mol during HS6. Interestingly, the response of Mg/Ca ratios diverge
from that of the stable isotopes during the deglaciation, with an
increase from 27 to 40 mmol/mol (Fig. 2). While this could potentially
indicate that PCP increased during the Holocene-Pleistocene transi-
tion, the δ13Candδ18O evidence both point towards wetter conditions,
consistent with regional records22,46,50, which should lead to less PCP
rather than more. We therefore suggest the increasing Mg/Ca trend
may instead reflect the influence of temperature on Mg partitioning
into calcite51,52 (see SI Note 5). During the glacial period, Mg/Ca data is
consistent with a PCP control, especially during HS2-6, as evidencedby
asignificant positive correlation between δ13CandMg/Ca
(r=0.51–0.77, p≤0.01). We therefore interpret higher Mg/Ca ratios
and enriched 13Ctoreflect enhanced PCP during periods of reduced
local water balance (See SI Note 4).
Potential forcings of precipitation on orbital timescales
Orbital-driven variations in insolation have been invoked to explain
widespread moisture variations in the broader region of
Mesoamerica22, as well in NE Mexico14, but most of these records only
span one precession cycle. The CB2 record, which spans ~2.5 preces-
sion cycles and extends ~25,000 years beyond the oldest lake record
from NE Mexico30, thus offers a unique opportunity to further con-
strain the impacts of insolation on precipitation and local water bal-
ance. While summer insolation has been proposed to drive
precipitation in NE Mexico via a northward shift in the ITCZ and a
strengthening of the CLLJ14,andinNWMexicoviaanintensified
NAM17,18, our record does not demonstrate a strong correlation to
summer insolation. However, our record is not alone in that a growing
number of records across Mesoamerica have also found a weak cor-
relation to NHSI, suggesting the influence of autumn, winter, or spring
insolation as the dominant driver of hydroclimate variability on orbital
timescales. For instance, Roy et al.15,31,53, have attributed a strong cor-
relation between increased watershed erosion and/or runoff in north-
ern Mexico to autumn insolation through increased tropical cyclone
and hurricane activity. Furthermore, water scarcity in North Central
Mexico recorded by increased authigenic calcite precipitation in a
sediment core from ephemeral Lake Santiaguillo, has been linked to
peaks in spring insolation16. Even winter insolation has been linked to
an extended wet season in speleothem δ18O records from Santo Tomás
Cave in Cuba and Terciopelo Cave in Costa Rica46,54. However, com-
parison of CB2 δ18O to Northern Hemisphere insolation from different
seasons demonstrates consistently weak correlations over the last ~57
ka (Fig. S6). While CB2 δ18O seemingly changes with insolation over the
last 20,000 years (Fig. 3, Fig. S6), exhibiting an in-phase response with
autumn insolation or a lagged response to summer insolation, this
relationship does not continue throughout the late-Pleistocene. This
lack of a consistent insolation pattern suggests that other factors such
as global ice volume, radiative forcing from atmospheric pCO
2
,orSSTs
may play a more direct role in explaining glacial-interglacial hydro-
climate variability in NE Mexico compared to insolation alone.
The CB2 δ18O time series shows much stronger similarity to
regional and global temperature records, indicating that precipitation
at our study site may be more sensitive to thermodynamic controls on
orbital timescales. Comparison with the Greenland ice core δ18O
record (ref. 55; Fig. 3), shows the CB2 record is dominated by relatively
cool and/or dry glacial conditions, as evidenced by relatively positive
δ18O values from 62.5 to 20 ka, with a shift towards warmer and/or
wetter conditions during the deglacial and Holocene. Superimposed
on this orbital-scale trend are millennial scale shifts towards more
positive δ18O values, indicating even drier conditions during Heinrich
Stadials and the Younger Dryas. These trends are reproduced by the
CB2 δ13C and Mg/Ca records. The CB2 record exhibits a much closer
relationship with atmospheric pCO
2
than with insolation (r=−0.61,
p<0.05, Fig. 3b).
However, the observed correlation between δ18OandpCO
2
is
primarily associated with the deglaciation, evident in the large lag
times, different topology, and an insignificant correlation over the
glacial period between 20–62.5 ka (r=−0.41, p= 0.11).
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In contrast to insolation and pCO
2
,CB2δ18O exhibits a much
better correlation (r=−0.49 to −0.73) with regional SSTs, including the
Tropical North Atlantic, the Gulf of Mexico, the Caribbean Sea, and the
Tropical Eastern Equatorial Pacific(Fig.3c, Fig. S756–59). Other records
from the region have also shown a strong correlation to changes in
local SSTs on orbital timescales. For instance, increased rainfall in SE
USA indicated by increased Mississippi River discharge has been
attributed to warmer SSTs60. Also, stalagmite records from Cuba,
Guatemala, and Puerto Rico have also attributed precipitation varia-
bility to Gulf of Mexico and Caribbean Sea SSTs on glacial-interglacial
timescales46,61,62. Even lake sediment records from NE Mexico have
identified SSTs asan important driver of climate on orbital timescales,
but this record did not maintain a strong correlation to SSTs into the
Holocene, consequently, more emphasis was placed on insolation,
shifts in the ITCZ, and a stronger CLLJ14.
While all SST records co-vary with CB2 δ18O on orbital timescales,
only the Tropical Pacific and Atlantic SSTs exhibit similar variability on
millennial timescales. The lack of millennial scale variability in some
records is possibly driven by slow sedimentation rates58, seasonal
biases59, or intra-Gulf of Mexico SST variability from Mississippi River
discharge or loop current strength63. Regardless, the CB2 δ18Orecord
suggests precipitation is responsive to broad-scale, basinwide changes
in Tropical North Atlantic and Eastern PacificSSTs.
Previous work has attributed cooler Tropical North Atlantic SSTs
to reduced precipitation in southern Mexico primarily by reducing
boundary layer moisture and convective activity28,however,warmer
Atlantic SSTs were thought to decrease moisture transport to North-
ern Mexico from a weakened inter-basin pressure gradient and CLLJ23.
The CB2 record, instead, demonstrates precipitation responds simi-
larly to southern Mexico via a positive correlation to Atlantic SST
variability. This finding is consistent with a recent interannual spe-
leothem record from the Common Era, which also suggests cooler
Atlantic SSTs drive decreased precipitation in theregion9.Additionally,
the sensitivity of CB2 δ18O to PacificSSTs(Fig.3) has been shown in
other records to alter precipitation throughout Northern Mexico,
primarily through shifts in the sub-tropical jet stream and reductionsin
winter storms9,64,65. We therefore suggest glacial-interglacial variations
in sea surface temperatures, indirectly reflecting orbital forcing and
associated feedbacks, are the most important driver of precipitation
variability in NE Mexico on orbital timescales.
Millennial-scale droughts linked to strengthened CLLJ and
lower SSTs
While previous records have suggested a variable response of North-
ern Mexico precipitation to millennial-scale AMOC changes30,66,CB2
proxies consistently record drying in northeast Mexico during Hein-
rich Stadials, with the exception of HS 2 (Fig. 2). Roy et al.14 proposed
that a southward shift of the ITCZ drove a weakening of the CLLJ
during Heinrich Events, thus decreasing the northward moisture
transport and subsequently leading to dry conditions in NE Mexico.
However, on seasonal timescales, the CLLJ actually strengthens when
the ITCZ shifts south during Borealwinter. Furthermore, other records
in southern Mesoamerica and the circum-Caribbean region point
towards SSTs as a more important influence on regional
hydroclimate46,61,62. Clearly, a better understanding of the dynamical
and thermodynamical influences on precipitation change during
Heinrich Stadials is needed in this region.
To evaluate the underlying climate dynamics associated with
drying during Heinrich Stadials, we analyzed results of an isotope-
enabled Earth System Model simulation (iCESM1;67) forced with
freshwater added to the North Atlantic on a glacial background state
(See methods). A comparison of pre-industrial iCESM1 precipitation to
merged model-observation data from the Global Precipitation Clima-
tology Project (GPCP) demonstrates iCESM1 correctly replicates the
overall pattern of precipitation in Mesoamerica, but overestimatesthe
magnitude of precipitation change in the Eastern Tropical Pacific
(Fig. S8). Previous work hasdemonstrated this is likely driven by model
sensitivity to orography and poorly resolved topography in Central
America, however, the response of precipitation in higher subtropical
latitudes has been shown to more closely match simulations68.Our
discussion will focus on the underlying dynamics driving patterns of
precipitation change rather than specific changes in precipitation
amount (i.e. mm/day).
Model results show significant SST cooling in the North Atlantic
(Fig. 4a) during Heinrich Stadials, relative to the LGM. The magnitude
of cooling is large but is supported by realistic proxy-based estimates.
For instance, two sediment cores reconstructing summer SSTs from
monospecific planktonic foraminifera suggest upwards of 10 degrees
of cooling during HS1 compared to the LGM69, and annual SST simu-
lations of the Tropical North Atlantic which show more moderate
cooling (2–4 °C) are in close agreement with regional SSTs recon-
structions (Fig. 3). Cooling is simultaneously driven by a decrease in
heat transport by a weakened western Gulf Stream and the positive
wind-evaporation-SST feedback via an enhanced Bermuda High70.
Moreover, cooling in the tropical Atlantic (~6 °C) is considerably
stronger than cooling in the tropical Pacific (~1 °C), creating an East-
West inter-basin temperature and sea level pressure (SLP) gradient
(Fig. 4a). A stronger Bermuda High results in an intensification of the
tropical easterlies which funnel into the Caribbean Sea (Fig. 4c), and
the inter-basin temperature and sea level pressure gradient further
magnify the flow of winds across the Isthmus of Tehuantepec (Fig. 4c).
However, unlike the seasonal intensification of the tropical easterlies
leading to increased rainfall, resulting wind anomalies combine with
considerably cooler SSTs to significantly reduce vertically integrated
moisture flux (Fig. 4c) and precipitation amount (Fig. 4b) over most of
Mesoamerica. While circulation changes likely modify precipitation
elsewhere in North America, we focus on modeled precipitation
changes solely in Mesoamerica, where changes are statistically sig-
nificant (Fig. 4). This is consistent with other hosing experiments,
which show divergent responses of rainfall to North Atlantic fresh-
water forcing71.
The modeled reduction in regional precipitation (Fig. 4b) is cou-
pled withan increase in precipitation δ18OinNEMexico(Fig.4d), which
Fig. 3 | Comparison of CB2 δ18O to various potential forcings. a Autumn (SON,
r=−0.48, p<0.14, orange) and Summer (JJA, r=−0.26 p= 0.46 black) insolation.
bAtmospheric pCO
2
(r=−0.61, p<0.01, maroon109). cSSTs from Gulf of Mexico
(r=−0.52, p<0.03, silver
59), Caribbean (r=−0.59, p<0.05,limegreen
58), Tropical
N. Pacific(r=−0.55, p<0.03,magenta
56) and Tropical N. Atlantic (r=−0.73, p<0.01,
teal57). dGreenland temperatures (r=−0.61, p< 0.01, indigo110).
Article https://doi.org/10.1038/s41467-023-37700-9
Nature Communications | (2023) 14:2279 5
Content courtesy of Springer Nature, terms of use apply. Rights reserved

is reasonably consistent with the 1.5‰increase observed in
CB2 speleothem δ18OduringHS1(Fig.2). The freshwater added to the
model experiment (0.5 Sv for 100 years) is most similar to the fresh-
water released during HS 147, upwards of 66% more than during other
Heinrich Stadials and explains why we see a stronger drying during HS1
than during other stadials. Although through different mechanisms,
the Younger Dryas is estimated to exhibit similar SST cooling as
Heinrich Stadials, explaining the similarmagnitude of drying indicated
by δ13C. Moreover, the CB2 record suggests that changes in AMOC
work in both directions, with increased precipitation leading to a −2‰
excursion in δ18O, a −3‰shift in δ13C and, a decrease of 15 mmol/mol of
Mg/Ca during the Bølling-Allerød when AMOC was strengthened and
SSTs are warmer72.
In juxtaposition to previous proxy studies from Northern Mexico
which speculated a weakening of tropical easterlies and the CLLJ in
response to Heinrich Stadials, our climate model results demonstrate
tropical easterlies in fact strengthened during reductionsin AMOC and
a southward shifted ITCZ, similar to the dynamical response of the
modern CLLJ during boreal winter. Moisture budget analysis suggests
drying in southern Mesoamerica is primarily driven by the strength-
ening of the easterlies and an intensified zonal inter-basin SST and SLP
gradient causing stronger winds across the Isthmus of Tehuantepec
and a southward migration of moisture convergence (Fig. S9). This
reduction in precipitation is part of a larger scale response in which an
inter-hemispheric SST and SLP gradient leads to anomalous cross-
equatorial winds and a southward shift in ITCZ precipitation. Although
models currently do not capture tropical cyclones, stronger low-level
winds cause an amplification of vertical wind shear and is therefore
also likely associated with a reduction in transient weather events,
including tropical cyclones and hurricanes9,13. Stronger tropical east-
erlies also significantly reduce Tropical Atlantic SSTs through the
positive wind-evaporation-SST feedback loop. The reduction in Tro-
pical Atlantic SSTs is alsoknown to further reduce summer convection
in the region73, thereby leadingto thermodynamic driven reductions in
precipitation throughout regions of Northern Mexico (Fig. S9). We
therefore demonstrate the combination of stronger easterlies and
cooler SSTs during Heinrich Stadials are particularly important for
reducing convection in NE Mexico and highlight the possible
mechanisms in which colder SSTs could contribute to the strong
similarities observed between CB2 δ18O and regional SST reconstruc-
tions on millennial to orbital timescales.
Our work also suggests thatprecipitation in northern Mexico is in
phase with southern Mesoamerica and the Caribbean. We find CB2
exhibits similar patterns of variability to speleothem δ18Orecordsin
Costa Rica54 and Cuba46, Cariaco Basin sediment records74,andmag-
netic susceptibility records from Guatemala50 on millennial timescales
(Fig. 5). Widespread regional drying during Heinrich Stadials has pre-
viously been attributed to a southward displaced ITCZ and/or cooler
Fig. 4 | iCESM1 simulation of Heinrich Stadials compared to LGM over North
America. a Annual temperature and Sea Level Pressure changes. bAnnual pre-
cipitation change, with statistically significant changes contoured by dashed gray
lines. cAnnual changes in total column precipitable water and 850 mb winds.
dAnnual changes in the stable oxygen isotope ratio of precipitation.
Article https://doi.org/10.1038/s41467-023-37700-9
Nature Communications | (2023) 14:2279 6
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SSTs in response to a weakening of AMOC46,61,62,75,76. However,previous
work has cautioned against extrapolating paleo precipitation records
to large-scale inter-hemispheric atmospheric changes, such as mer-
idionalshifts in the ITCZ, because data synthesisand modeling analysis
suggests a regionally variable precipitation response62,77.
Through the addition of our paired proxy and climate model
analysis, we suggest the importance of the more localized Atlantic-
Pacific inter-basin SST and SLP gradient on driving easterly wind
anomalies, which subsequently reduces local SSTs,vertical wind shear,
and convective activity in the region. The culmination of these pro-
cesses drives moisture convergence to shift southward, as supported
by our modeling results and previous paleo records78. Ultimately, the
intensified inter-basin gradient causes spatially ubiquitous drying
across Mesoamerica and the Caribbean (Figs. 4,5). Although cold
events like Heinrich Stadials are never perfect analogs for future pre-
cipitation change, these new results suggest precipitation across
Mesoamerica and the Caribbean is highly sensitive to the Atlantic-
Pacific SST gradient, and could exhibit a similar broadly coherent
response to changes in AMOC in the future.
Wet conditions in NE Mexico during the Last Glacial Maximum
and HS 2
The LGM is an important time period for highlighting mechanisms of
precipitation change and evaluating climate model performance5,79.
The CB2 record provides an additional record of high-resolution pre-
cipitation change before, during, and after the LGM, and is well suited
for future model-proxy comparison studies. In contrast to the other
Heinrich Stadials, the CB2 δ13C and Mg/Ca records suggest an exten-
ded period of increased local water balance at our study site during HS
2(~24ka;Fig.2), just before the LGM. This is in contrast to other proxy
records from Lake Péten Itzá, Guatemala50,80, Santo Tomás Cave,
Cuba46,andTerciopeloCave,CostaRica
54, which all show the expected
response of drying during HS 2 (Fig. 5). While the CB2 response could
potentially reflect a highly localized signal or be impacted by non-
climatic proxy controls, the clear covariation between δ13CandMg/Ca
during this event does suggest it was characterized by increased water
balance. There are three potential mechanisms we consider that could
explain the wetter conditions in NE Mexico during HS 2, which occurs
around the time of the LGM: (1) Increased winter precipitation derived
from the Pacific winter storm track during the LGM, (2) A weaker HS 2
event which NE Mexico hydroclimate is more sensitive to than other
regions, and/or (3) increased water balance due to colder tempera-
tures during HS 2 and the LGM.
An intensified Pacific winter storm track during the LGM has been
frequently evoked to explain increased precipitation across the Great
Basin, Southwest US, and Northern Mexico, but the spatial extent of
the winter storm track has been previously disputed81,82. Precipitation
sourced from the Pacific would have an isotopic composition heavily
depleted in 18O due to the more distal moisture source83, which is not
observed in CB2 δ18O during this time period. In fact, speleothem δ18O
demonstrates no significant change during HS 2 while both δ13Cand
Mg/Ca ratios (Fig. 2) decrease, suggesting anomalous increases in local
water balance. PMIP3 model simulations of the LGM further support
this interpretation, showing an increase in the magnitude of the CLLJ
but no increase in winter precipitation (Figs. S10, 11). Therefore, we
rule out contribution of rainfall from an enhanced winter storm track
in NE Mexico during HS 2 or the LGM.
There is still some debate about whether HS 2 resulted in a sig-
nificant AMOC reduction similar to other Heinrich Stadials. Higher Pa/
Th ratios in North Atlantic sediment cores are traditionally interpreted
to be reflective of a weakened AMOC49. However, increases in opal flux
as noted during HS 2 and HS 3 could also contribute to higher Pa/Th
ratios49. Additionally, oxygen isotopes from benthic foraminifera in
cores from the Caribbean suggest changes in oceanic circulation
during HS 2 were lower in magnitude compared to those during HS 1
and the YD49. A stronger than anticipated western gulf stream, as
potentially indicated from Caribbean benthic foraminifera, would have
helped mitigate SST cooling, leading to less severe drying in NE Mex-
ico. This idea is further supported from SSTs from the Gulf of Mexico,
the North Atlantic and the Caribbean,that show elevated temperatures
compared to other Heinrich Stadials. Therefore, thelack of response of
CB2 δ18O to HS 2 in our proxy record lends further support to the
possibility that HS 2 may have been weaker than other Heinrich
Stadials.
Finally, we suggest the decrease in δ13C and Mg/Ca ratios during
HS 2 and the LGM may be driven by decreased temperature at our
study site, inhibiting evapotranspiration (ET), and leading to a higher
local water balance (P-ET). We suggest precipitation remained rela-
tively constant as suggested by stable δ18O during this time-period.
PMIP3 models also support this interpretation, where increased local
water balance is observed in elevated soil moisture content in the
LGM, compared to both the Mid-Holocene and Pre-Industrial period,
even where models consistently disagree on both the sign and
magnitude of precipitation change (Fig. S12). Increased soil moisture
is likely driven by a reduction in ET, linked to increased cloudiness84,
reduced land temperatures85, or changes in relative humidity over
land86. While evaluating the exact mechanism of increased local
water balance is beyond the scope of this paper, our record is con-
sistent with PMIP3 data and recent modeling studies87, demonstrat-
ing high lake stands in Mexico and Central America50 may have been
driven by decreased ET.
Fig. 5 | Comparison of CB2 with otherpaleoclimate records. Comparison of CB2
δ13O(blue)andδ13C (orange) to Pa/Th ratios(burgundy; Henryet al. 111) with Florida
Strait gulf stream circulation (magenta49), Cariaco Basin reflectance (purple74),
Juxtlahuaca (lightgreen22), CostaRica (blue108), Cuba(dark green46) and LakePetén-
Itzá (black50).
Article https://doi.org/10.1038/s41467-023-37700-9
Nature Communications | (2023) 14:2279 7
Content courtesy of Springer Nature, terms of use apply. Rights reserved

Implications for climate model simulations
We have presented the first multi-proxy speleothem record from NE
Mexico that highlights millennial and orbital scale hydroclimate
variability from 62.5 to 5.1 ka. In contrast to other speleothem records
from tropical and monsoon regions, we find no strong evidence for a
direct insolation control on regional hydroclimate. We show instead
that glacial-interglacial variations are closely linked to Atlantic and
Pacific SST variations on orbital timescales. On millennial timescales,
we find dry conditions in NE Mexico during the Younger Dryas and HS
1, 3–6. We utilize an isotope-enabled climate model to show that
Heinrich Stadial drying is dominantly driven by significant reductions
in Atlantic SSTs and a strengthening of tropical easterlies,which drives
a strong Atlantic-Pacific SST/SLP gradient and a southward shift in
moisture convergence. Comparison to records from southern
Mesoamerica and the Caribbean suggests this mechanism leads to a
spatially broad and large magnitude precipitation change in response
to weakened AMOC during Heinrich Stadials. However, we find evi-
dence for increased local water balance during HS 2 and the LGM,
which we suggest reflects a combination of a relatively weaker HS 2 and
reduced evapotranspiration at our study site.
CB2 provides a much-needed record of precipitation that can be
useful for model validation. Our results suggest an important role for
both strengthened tropical easterlies, which feed in to the CLLJ, and
cool SSTs. The combined influence decreases moisture transport to
the region during millennial-scale cold events—indicating that both
dynamical and thermodynamical mechanisms are important drivers of
hydroclimate in NE Mexico. Furthermore, the disagreement about the
magnitude of drying in Mesoamerica is largely due to inter-model
spread in Atlantic-Pacific SST gradients29. Records of past climates,
such as CB2, which include key intervals such as the LGM, Heinrich
Stadials, and Pleistocene-Holocene transition, provide critical con-
straints for model simulations of the interbasin gradients and their
respective impact on hydroclimate. This work, along with Wright et al.9
and Bhattacharya and Coats29, emphasizes how a better understanding
of the trend and variability in the Atlantic-Pacific inter-basin gradient
will help generate more reliable predictions of future rainfall in
Mesoamerica and the Caribbean.
Methods
Chronology
The CB2 stalagmite was cut, polished and sampled for 33 U-Th dates at
2.5 cm intervals along its vertical growth axis using a Dremel hand drill
with a diamond dental bur. The CB2 sample has uranium concentra-
tions ranging from 18 to 63 ng/g (Table S1). Calcite powder samples
weighing 250–300 mg were prepared at Massachusetts Institute of
Technology following methods similar to Edwards et al., (1987)88.
Powders were dissolved in nitric acid and spiked with a 229Th –233U–
236U tracer, followed by isolation of U and Th by iron co-precipitation
and elution in columns with AG1-X8 resin. The isolated U and Th
fractions were analyzed using a Nu Plasma II-ES multi-collector
inductively coupled plasma mass spectrometer (MC-ICP-MS) equip-
ped with an Aridus 2 desolvating nebulizer, following methods
described in Burns et al.89. The corrected ages were calculated using an
initial 230Th/232Th value of 10.5 ± 5.3 ppm to correct for detrital 230Th.
The initial 230Th/232Th value wasdetermined by testing dates corrected
with different initial 230Th corrections for stratigraphic order and
assuming a ± 50% uncertainty, similar to methods by Hellstrom90,
Cheng (2000)91,andLin(1998)
92. The stratigraphically determined
value of 10.5 ppm closely matches the measured initial Th value of 9.5
ppm of a modern speleothem sample from Cueva Bonita (CB4), which
was determined by matching various U-Th dates with the rise in the
radiocarbon bomb peak (Fig. S13, Table S2). Although the initial Th
value of 10.5 ± 5.3 ppm is considerably higher than the traditional
assumed correction of 4.4 ± 2.2 ppm, previous work has demonstrated
select caves can have considerably higher initial 230Th/232Th values. For
instance, speleothems contaminated from detrital limestone, rather
than detrital shale predominantly composed of aluminosilicate clays,
have been shown to have initial 230Th/232Th values as high as
56–111 ppm93.
U-Th ages range from 5230 ± 3200 to 57800 ± 3900 years before
present (where present is 1950 CE), and all 33 dates fall in stratigraphic
order within 2σuncertainty (Table S1). The 95% confidence interval for
the age-depth model was constructed using 2000 Monte-Carlo simu-
lations through the age-depth modeling software COPRA33.Agemod-
els constructed with various 230Th/232Th values demonstrate age
uncertainties increase with larger initial Th values.But due to the use of
COPRA and abundance of Monte-Carlo simulations, the uncertainties
in the age model areconstrained, and do not scale proportionally with
the larger uncertainties in the U-Th dates from higher initial Th values
(Fig. S14).
Although we suggest the sample grew continuously from 5117 to
62525 years before present, there are some minor shifts in growth rate,
such as near the beginning of the Holocene between ~11 and 6 ka where
we have a gap in dating. While it is possible the slightly slower growth
rate during this period (~11 μm/yr) reflects a minor growth hiatus
during this time-period, the rate is only slightly lower than the mean
growth rate for the sample (~14 μm/yr). Furthermore, we find no
anomalous fabric changes in this part of CB2 so think it is reasonable to
assume it grew continuously during this period. Nevertheless, due to
the gap in dating between 11–6 ka and related uncertainty of the age
model at this time, the Holocene component of our record is solely
utilized as a point of comparison for examining glacial/interglacial
differences and we do not interpret it in detail. We also note a
decreased growth rate between 30,000 and 37,000 years before pre-
sent, where the age model exhibits the largest uncertainty, which
could potentially be indicative of a second growth hiatus. However,
there is no evidence of a change in calcite fabric or color during this
interval and 3 U-Th dates suggest the speleothem could have grown
continuously, when age uncertainties are accounted for. We note the
timing of millennial scale events discussed in this manuscript are
independently constrained by other U-Th dates and arenot dependent
upon the continuous growth of the sample during this time interval.
Precipitation δ18O and cave monitoring
The closest precipitation stations of the International Atomic Energy
Agency Global Network for Isotopes in Precipita tion (Veracruz and Sa n
Salvador) are over 600 km from our field site and likely do not
represent local patterns at the cave. Therefore, we established a local
precipitation collection station directly at our field site (Alta Cima). To
reduce kinetic effects due to evaporation, evaporation-limiting pre-
cipitation collectors were built and deployed during fieldwork
(Fig. S2). Samples were collected after rainfall events in 1.5 ml glass
vials with conical inserts, sealed with parafilm and refrigerated to limit
evaporation. In total, 48 samples were collected from June 2018 to May
2019 and analyzed for δDandδ18O using a Picarro L2130i cavity ring-
down spectrometer at Chapman University (Table S2). The long-term
standard deviation of an independent quality control sample is 0.51‰
δ2Hand0.11‰δ18O (VSMOW).
Moisture sourceanalysis at Cueva Bonita was conducted using air
parcel back-trajectory simulations using the NOAA HYSPIT model94
(Fig. S2). Air parcel trajectories were launched every 6 h at an elevation
of 1500 m between 2005 and 2018 using GDAS weekly data. Each tra-
jectory evaluated the air parcel’s location over the previous 72 h from
launch. In total, 3600 rain-bearing trajectories were produced for the
combined summer (JJAS) and winter (DJFM) months, only rain-bearing
trajectories were included in analysis (Fig. S2).
Stable isotope and trace element analysis
CB2 was micro-sampled for both stable isotope and trace element
analyses using a Sherline micromill at 500 μmincrementstoadepthof
Article https://doi.org/10.1038/s41467-023-37700-9
Nature Communications | (2023) 14:2279 8
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1 mm of the sample face, producing 1578 samples (Table S3). The
powder for CB2 was collected, weighed out to 40–80 μg and analyzed
on a Kiel IV Carbonate Preparation Device coupled to a Thermo Sci-
entific DeltaV-IRMS at the UC Irvine Center for Isotope Tracers in Earth
Sciences (CITIES) following methods similar to McCabe-Glynn et al.95
to determine δ18Oandδ13C95.Every32samplesofunknowncomposi-
tion were analyzed with 14 standards which included a mix of NBS-18,
IAEA-CO-1, and an in-house standard. The analytical precision for δ18O
and δ13C is 0.08‰and 0.05‰, respectively. Speleothem δ18Ovalues
were ice-volume corrected using mean ocean δ18Ovalues(Fig.S15).
For trace element analysis, 20–60 μg calcite powder aliquots
taken from the stable isotope samples were dissolved in 500 μLofa
double distilled 2% nitric acid solution. The samples were analyzed
using a Nu Instruments Attom High Resolution Inductively Coupled
Plasma Mass Spectrometer (HR-ICP-MS) at the CITIES laboratory. Mg/
Ca ratios were calculated from the intensity ratios using a bracketing
technique with five standards of known concentration and an internal
standard (Ge) added to all samples to correct for instrumental drift.
Trace element analysis ofCB2 serves to complement the interpretation
of speleothem δ18Oandδ13C; therefore, a lower-resolution (multi-
decadal to centennial) analysis was conducted over the complete
record by analyzing every other sample (789 total; Table S3). The full
data set reported in the supplementary materials is unsmoothed.
Earth system model simulations
We use a water isotope tracer enabled version of the Community Earth
System Model, iCESM167. Model physics are consistent with CESM1,
which simulates present-day and historical climate change quite well96.
Here, we run a fully-coupled configuration of iCESM1 with 1.9 × 2.5°
atmosphere (CAM5) and land (CLM4), and nominal 1° ocean (POP2)
and sea ice (CICE4). The model tracks stable water isotopes of oxygen
and hydrogen through all model components. Previous studies
demonstrate that the water isotope tracer components of iCESM1 can
accurately simulate the δ18O distribution of both present97 and past
climates98.
We configured our 21 ka simulation with period-appropriate
orbital parameters and greenhouse gas concentrations, as in the
PMIP4 protocol71, and ICE-6G ice sheets99. Initial isotopic distribution
in the ocean comes from the GISS interpolated ocean δ18Odataset
100
with globally uniform enrichment of +1 ‰for δ18O101. All other ocean
initial conditions come from a previously performed LGM
simulation102.Werunthismodelconfiguration for 500 years to reach
quasi-equilibrium, with our analyses coming from the final 50 years of
simulation. We then branch this simulation to explore the effects of
melt water flux in the North Atlantic atthe LGM. Starting from year-500
of the 21 ka simulation, we add 0.50Sv of freshwater with a δ18Oof
−30‰into the North Atlantic (50°N–70°N), sufficient to rapidly and
substantially weaken AMOC103. After 100 years, we s hutoff the fresh-
water flux and extend the simulation for another 50 years. Analyses
come from the final 50 years of the 150-year simulation.
We also perform a moisture budget analysis on the simulation
presented in Fig. 4, following the mathematical decomposition of
Seager & Henderson104. The thermodynamic term refers to changes in
P-E due to changes in specific humidity, while the dynamic term refers
to changes due to winds. Because the analysis isperformed on monthly
mean terms, a residual term, calculated by subtracting the full
response from the dynamic and thermodynamic terms, incorporates
the influence of higher-resolution terms (e.g. transient eddies) and
non-linear terms105.
Data availability
Speleothem stable isotope and trace element data generated in this
study are provided in the Supplementary Data 1. The radiocarbon, and
U-Th data generated in this study are provided in Supporting Infor-
mation. All data have also been deposited in the NOAA paleoclimate
database, accessible at https://www.ncei.noaa.gov/access/paleo-
search/study/37679.
Code availability
iCESM1 code is available at https://github.com/NCAR/iCESM1.2.Ver-
sion 1.15 of the COPRA depth-age modeling software utilized to build
the age model in this study is available at https://doi.org/10.5194/cp-8-
1765-2012.
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Acknowledgements
We thank Cheva Berrones-Benitez for her assistance with rainfall sam-
pling. We thank Jim Kennedy, Esteban Berrones, Corinne Wong, and
Chris Wood for their help with fieldwork. We thank Dachun Zhang, Jes-
sica Wang, Chris Wood, and Elizabeth Patterson for assistance with lab
work. We thank Crystal Tulley-Cordova for sharing the precipitation
collector design. This research was supported by a UC MEXUS-
CONACYT Collaborative Grant from the University of California Institute
for Mexico and the United States (UC MEXUS CN-16-120; K.R.J. and
L.B.O.), an MIT International Science and Technology Initiatives Mexico
Program (D.M.), National Science Foundation awards AGS-1804512 and
AGS-1806090 (K.R.J. and D.M.), and UC National Laboratory In-
Residence Graduate Fellow award LGF-19-600874 (K.T.W.).
Author contributions
K.T.W., K.R.J., D.M., and L.B-O. designed the study; K.T.W., G.S.M, K.R.J.,
and J-L.L.M. conducted fieldwork; K.T.W., G.S.M., G.R.G., and G.L. con-
ducted laboratory analyses; C.R.T. conducted paleoclimate model
simulations; T.B., C.R.T., and K.T.W. analyzed model data; K.T.W. and
K.R.J. wrote the manuscript with help from coauthors. All authors con-
tributed to data analysis, interpretation and manuscript review.
Competing interests
The authors declare no competing interests.
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