PP33C-01 INVITED
Orbital influences on paleosalinity indicators
Water isotopes records collectively provide a set of the most extensive proxy evidence for past climate. Implied in the interpretation of these records is a set relationship between water isotopes and climate. In particular,the modern regional relationships between salinity and seawater δ18O are used to reconstruct paleosalinity. However, on orbital timescales and during abrupt climate change events, the climate goes under significant reorganization. These climate changes impact the hydrologic cycle, including water isotopes. As such, the relationship between water isotopes and climate is unlikely to remain constant through time. We assess the relationship between water isotopes and climate using different simulations of Holocene climate using GISS ModelE-R, a fully coupled atmosphere-ocean General Circulation Model equipped with water isotopes, as well as other tracers, making it ideal for performing exact comparisons with the proxy record of past climate change. We find that the relationships between water isotopes and salinity as well as surface air temperature and precipitation are different at decadal timescales than at orbital (millennial) timescales and that this relationship also changes during abrupt climate change events. In particular, the altering of the amount of water exported from the tropics, as well as exchanged between ocean basins, has profound impacts on the relationship between water isotopes and climate. Thus the relationship between water isotopes and climate is not constant through time.
PP33C-02 INVITED
El Nino Variability During the Holocene: Constraints From Individual Foraminifera
Interpretation of tropical hydrologic anomalies in paleoclimate records requires careful consideration of variability in the El Niño-Southern Oscillation (ENSO), as the latter interacts strongly with key hydrologic systems such as the monsoons and the Intertropical Convergence Zone (ITCZ). Here we present new insights into the Holocene evolution of ENSO from oxygen isotope distributions of individual G. ruber foraminifera from sediment core V21-30 south of the Galapagos Islands, in the heart of maximum ENSO activity. Several hundred δ18O analyses of individuals have been carried out in the Holocene section of the core, with more in progress. Our main findings are summarized as follows: All indicators of oxygen isotopic variance reveal a progressive increase in ocean variability at the core site since the middle Holocene with minimum variance present ~6 ky BP. The decrease in mid-Holocene variance is primarily driven by reduced frequency of extreme values associated with El Niño and La Niña events. We estimate that the mid-Holocene frequency of El Niño events was reduced by a factor of 4-5, and that of La Niña events by a factor of 2-3. The δ18O distribution shapes further suggest that the seasonal cycle of SST which is presently very strong, was weaker in the mid-Holocene, a plausible consequence of the altered seasonal insolation due to precession. The interaction of ENSO with the seasonal cycle is today a key element of interannual ocean-atmosphere dynamics in the tropical Pacific, and our results suggest that such a link has also been important in the longer Holocene evolution of the ENSO system. The results are further examined in the wider context of Holocene climate adjustments in the tropical Pacific region involving the Walker circulation, the East Asian Monsoon and the ITCZ.
PP33C-03 INVITED
The Atlantic-Pacific Salinity Gradient; A Variable?
Estimates of Glacial salinity change in the tropical-subtropical Atlantic are derived by measuring the δ18O of the surface-dwelling planktonic foraminifer Globigerinoides ruber (white) taken from a suite of box cores collected between 30o North and 30o South and solving the calcite δ18O paleotemperature equation for δ18O of seawater (δw), which varies systematically with salinity. Mg/Ca paleothermometry is used to derive SSTs used to solve the δ18O paleotemperature. The δw values are converted to salinity via the modern salinity-δ18O relationship. At sites where the water depth is less than 3600 meters the δ18O -Mg/Ca technique applied to G. ruber precisely estimates summer (not mean annual) sea surface temperatures and salinities in the Atlantic. In the late glacial the tropical and subtropical SSTs were an average of 2.5oC colder than modern and late Holocene values. The difference between late Holocene and LGM δw values is close to 1‰ at tropical sites located near the equator and therefore comparable to the ice volume effect. At subtropical sites the difference is much less than 1‰, varying from 0.6 to 0.0‰ in both the northern and southern hemisphere. The Holocene-LGM comparison implies that surface salinities in the subtropical gyres of both the north and south Atlantic were lower during the LGM than during the late Holocene. These results highlight a potentially important hydrologic feedback on the salinity structure of the Atlantic.
PP33C-04
Evolution of Tropical Atlantic SST Gradients Since the LGM and Associated Shifts of the Marine Atlantic ITCZ: A New Look Using New Salinity and Temperature Calibrations
Tropical Atlantic sea surface temperature (SST) and sea surface salinity (SSS) gradients are closely tied to the position of the modern marine Atlantic ITCZ. To reconstruct changes in tropical SST and SSS gradients since the LGM we measured paired G. ruber(w) Mg/Ca and d18O for six cores that form a transect with both a meridional and a zonal component across the tropical Atlantic. During the LGM, equatorial Atlantic SSTs were cooler by about 3°C and the modern zonal SST gradient was nearly absent, suggesting weaker trade winds on the equator. Following the conventional approach of solving for d18Osw using shell Mg/Ca and d18O measurements we find antiphase salinity histories north and south of the equator, documenting northward (southward) ITCZ shifts during the early Holocene (LGM). Using a larger Atlantic basin coretop calibration transect, we have recently discovered a significant salinity effect on shell Mg/Ca values that introduces significant d18Osw biases. Using new calibration equations relating paired shell Mg/Ca and d18O to estimate surface temperature and salinity (r2=0.8) we revisit our tropical Atlantic data and find that the principal conclusions regarding the temperature and salinity gradient changes are intact: we see comparable changes in the SST and SSS gradients and overall LGM cooling. These new data should represent a more accurate picture of the tropical Atlantic since the LGM as they correct for the observed "salinity effect" on Mg/Ca derived temperatures.
PP33C-05 INVITED
Rapid Amazonian Moisture Oscillations Correlated with Dansgaard-Oeschger Cycles
Terrestrial paleoclimate records on abrupt climate events from the tropics, e.g. Dansgaard-Oeschger (D-O) oscillations, are still rare, in particular, from Amazonia, which contains the largest tropical rainforest in the world. We have obtained a high-resolution oxygen isotopic record of cave calcite from Caverna Paraíso (PAR, 04o04'S, 55o27'W), Amazonia, Brazil. The chronology was determined by 69 U-Th ages from 4 stalagmites. Tests for equilibrium conditions show that their oxygen isotopic variations are primarily caused by climate change. We thus interpret the Paraíso record, spanning the last 50 thousand years, in terms of meteoric precipitation changes at this equatorial location. The oxygen isotopic profile shows significant abrupt millennial-scale variations during Marine Isotope Stage (MIS) 3, with amplitudes as large as 2 per mil. Using independent age scales, we compare the record to contemporaneous records from caves in eastern China and high-latitude ice cores. During MIS 3, the PAR calcite oxygen isotopic profile correlates remarkably with the Hulu Cave record (Wang Y.J. et al., 2001, Science), indicating that precipitation histories at the two sites are asynchronous, similar to our previous observations from northeastern and southern Brazil speleothems (Wang X.F. et al., 2004, Nature; Wang X.F. et al., 2006, Quat. Sci. Rev.). During MIS 3, Paraíso precipitation also broadly anti-correlates with Greenland D-O events (NGRIP members, 2004, Nature) and positively correlates with Antarctic warm events (EPICA community members, 2006, Nature). Our record adds further support to the idea that abrupt climate events have a worldwide distribution during MIS 3 (Voelker et al., 2002, Quat. Sci. Rev.). The observed correlations between the records support an oceanic meridional overturning circulation mechanism for driving the abrupt millennial-scale events of the last glacial period, coupled with strong air-sea feedbacks from the tropics. In combination with Andean ice core and lake records (e.g. Thompson et al., 2006, PNAS), knowledge of the meteoric precipitation oxygen isotope and moisture history of the central Amazon may shed new light on the role of the tropics in abrupt climate change.
PP33C-06
Linking Atmospheric and Ocean Circulation Change Across the Last Glacial Termination
In order to determine the relative timing between atmospheric vs. ocean circulation changes in the North Atlantic across last glacial termination, we generated proxy records for sea surface salinity (SSS) and ocean circulation change in the same low latitude North Atlantic sediment cores for the last 20 kyr. By combining Mg/Ca- paleothermometry with δ18O analyses of shells from the surface-dwelling foraminifera Globigerinoides ruber in Florida Straits cores KNR166-2-JPC29 (24°17'N, 83°16'W; 648 m; 9-19 cm/kyr sed. rate) and JPC26 (24°19.61'N, 83°15.14'W; 546 m depth; 18-240 cm/kyr sed. rate) we generated a record of δ18OSEAWATER (δ18OSW) change. After removing the δ18OSW signal due to continental ice volume variation (Fairbanks et al., 1992), the resulting ice volume-free δ18OSW record (a proxy for SSS variability due to regional hydrological change) shows that regional δ18OSW values increased by more than 0.6‰ during Heinrich Event 1 (H1) and the Younger Dryas (YD) and returned to modern values (about 1.0‰) during the Bolling Allerod and at the end of the YD. Based on the modern tropical Atlantic δ18OSW:SSS relationship, δ18OSW=0.26*SSS-8.44, a 0.6‰ enrichment in δ18OSW equates to a 2.3 increase in SSS. In comparison, benthic Cibicidoides pachyderma δ18O values in the same cores decrease during the YD and H1, suggesting an intermediate-depth warming on the western margin of the Florida Straits. Benthic δ18O values on the eastern margin of the Florida Straits do not show a coeval warming, indicating that the cross-current density gradient was relaxed in response to reduced geostrophic transport through the Florida Straits. The abrupt benthic δ18O decrease at the start of the YD is synchronous with an increase in ice volume-free G. ruber δ18O values and with calculated ice volume-free δ18OSW values, suggesting the rapid development of increased regional SSS in less than 100 years. Such a rapid change suggests that an atmospheric, rather than an oceanic mechanism is responsible for the increase in regional SSS at the start of the YD. On the transition out of the YD, benthic δ18O values in JPC 26 suggest a somewhat gradual increase in Florida Current transport from 12 to 11.4 kyr. In comparison, the corresponding ice volume-free G. ruber δ18O values also begin to return to pre- YD values around 12 kyr, but complete the transition before benthic δ18O values, suggesting atmospheric circulation returned to a pattern characteristic of the early Holocene before meridional overturning circulation fully recovered.
PP33C-07
Multiproxy Deglacial Record of Climate Change in Central Florida
An 11.28 m core from Tampa Bay, Fl (MD02 2579) reveals lacustrine sediments during the deglacial warming. Nineteen AMS radiocarbon dates were used to establish the chronology. Mg/Ca, Sr/Ca and 18O were measured on two species of brackish water ostracodes, Candona annae and Limnocythere floridensis. Variations in Mg/Ca and Sr/Ca reflect changes in water chemistry, which is related to evaporation/precipitation (E/P) or net moisture. Higher Me/Ca values imply increased evaporation and/or reduced precipitation during drier climates and vice-versa. Downcore trends for both elemental ratios and for both species are remarkably similar. Minor element ratios decrease by 20-30% from 20 to 17 ka, reflecting drier conditions during the LGM. High values at 14.3 ka decrease dramatically to minimum values at 14.1 ka suggesting moist conditions at 14.1 ka. These ratios increase by a factor of two from 14.1 to 12.9 ka with the driest conditions at the onset of the Younger Dryas (YD). Me/Ca ratios decrease by c. 20 % from 12.9 to 11.5 ka indicating increasing moisture during the YD. Millennial and centennial scale changes in δ 18O are consistent with changes in net moisture recorded in Me/Ca as well as long-term changes in ice volume. Neither orbitally induced variations in seasonal insolation or documented shifts in the ITCZ can explain the data. Changes in the meridional heat transport as a consequence of changes in thermohaline circulation explain the wetter YD. Reduced THC leads to cooler N. Atlantic and warmer/wetter Florida. Our ostracode results, which reflect climate at the same time the shell is precipitated, are consistent with pollen data from the same core (Willard et al, 2007), with a lag of 180 years. This lag is consistent with theoretical predictions that vegetation changes should lag a stepped climate event by 100-200 years.
PP33C-08
A 1400 Year Multi-Proxy Record of Hydrologic Variability in the Gulf of Mexico: Exploring Ocean- Continent Linkages During the Late Holocene
The timing and phasing of variations in hydrologic conditions between the North American (NA) continent and the Gulf of Mexico (GOM) during the Late Holocene are critical to understanding current and future responses to natural and anthropogenic climate changes. The Pigmy Basin in the northern GOM is ideally situated to record inputs from the Mississippi River (MR), and, thus, can provide a coherent, decadal-scale assessment of oceanic and continental responses to changing hydrologic conditions over NA and in the GOM. This study focuses on the sedimentary record spanning the last 1400 years and utilizes a multi-proxy approach incorporating organic and inorganic geochemical analyses to define intervals of varying continental inputs and to assess changes in the moisture balance (E/P) within the GOM. Results show multi-decadal episodes of significantly increased terrestrial inputs and enhanced MR discharge (flood intervals) centered at 1500 and 1000 A.D. that coincide with intervals of increased δ18O seawater indicative of higher salinity recorded in the Pigmy Basin. Episodes of lower continental inputs (i.e. drought intervals) at 1850, 1150 and 600 A.D. are associated with decreased salinity in the GOM. Since the continental inputs and salinity variations are synchronous, we hypothesize that hydrologic variability recorded over the NA continent is directly dependent on the moisture balance (E/P) over the sub-tropical GOM (a major source of moisture to the NA continent). For example, increased evaporation over the GOM leads to enhanced precipitation over the NA continent, due to the intensification of the atmospheric Hadley cell circulation, which influences meridional moisture flux from the GOM to the NA continent. Additional regional marine and terrestrial hydrologic records from the sub-tropics and the NA continent are in strong agreement with these hydrologic records, further supporting the hypothesis.