Lynch-Stieglitz
Jean
Lynch-Stieglitz
Jean
No Thumbnail Available
10 results
Search Results
Now showing
1 - 10 of 10
-
ArticleFlorida Straits density structure and transport over the last 8000 years(American Geophysical Union, 2009-09-01) Lynch-Stieglitz, Jean ; Curry, William B. ; Lund, David C.The density structure across the Florida Straits is reconstructed for the last 8000 years from oxygen isotope measurements on foraminifera in sediment cores. The oxygen isotope measurements suggest that the density contrast across the Florida Current increased over this time period. The magnitude of this change corresponds to an increase in the geostrophic transport referenced to 800 m water depth of 4 sverdrups (Sv) over the last 8000 years. The spatial and seasonal distribution of incoming solar radiation due to changes in the Earth's orbit has caused systematic changes in the atmospheric circulation, including a southward migration of the Intertropical Convergence Zone over the last 8000 years. These changes in atmospheric circulation and the associated wind-driven currents of the upper ocean could readily account for a 4 Sv increase in the strength of the Florida Current. We see no evidence in our data for dramatic changes in the strength of the Atlantic Meridional Overturning Circulation over this time period.
-
ArticleCalibration of the carbon isotope composition (δ13C) of benthic foraminifera(John Wiley & Sons, 2017-06-03) Schmittner, Andreas ; Bostock, Helen ; Cartapanis, olivier ; Curry, William B. ; Filipsson, Helena L. ; Galbraith, Eric D. ; Gottschalk, Julia ; Herguera, Juan Carlos ; Hoogakker, Babette ; Jaccard, Samuel L. ; Lisiecki, Lorraine E. ; Lund, David C. ; Martínez Méndez, Gema ; Lynch-Stieglitz, Jean ; Mackensen, Andreas ; Michel, Elisabeth ; Mix, Alan C. ; Oppo, Delia W. ; Peterson, Carlye D. ; Repschläger, Janne ; Sikes, Elisabeth L. ; Spero, Howard J. ; Waelbroeck, ClaireThe carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.
-
ArticleRemineralization dominating the δ13 C decrease in the mid-depth Atlantic during the last deglaciation(Elsevier, 2021-07-20) Gu, Sifan ; Liu, Zhengyu ; Oppo, Delia W. ; Lynch-Stieglitz, Jean ; Jahn, Alexandra ; Zhang, Jiaxu ; Lindsay, Keith ; Wu, Lixinδ 13 C records from the mid-depth Atlantic show a pronounced decrease during the Heinrich Stadial 1 (HS1), a deglacial episode of dramatically weakened Atlantic Meridional Ocean Circulation (AMOC). Proposed explanations for this mid-depth decrease include a greater fraction of δ 13 C -depleted southern sourced water (SSW), a δ 13 C decrease in the North Atlantic Deep Water (NADW) end-member, and accumulation of the respired organic carbon. However, the relative importance of these proposed mechanisms cannot be quantitatively constrained from current available observations alone. Here we diagnose the individual contributions to the deglacial Atlantic mid-depth δ 13 C change from these mechanisms using a transient simulation with carbon isotopes and idealized tracers. We find that although the fraction of the low- δ 13 C SSW increases in response to a weaker AMOC during HS1, the water mass mixture change only plays a minor role in the mid-depth Atlantic δ 13 C decrease. Instead, increased remineralization due to the AMOC-induced mid-depth ocean ventilation decrease is the dominant cause. In this study, we differentiate between the deep end-members, which are assigned to deep water regions used in previous paleoceanography studies, and the surface end-members, which are from the near-surface water defined from the physical origin of deep water masses. We find that the deep NADW end-member includes additional remineralized material accumulated when sinking from the surface (surface NADW end-member). Therefore, the surface end-members should be used in diagnosing mechanisms of changes. Furthermore, our results suggest that remineralization in the surface end-member is more critical than the remineralization along the transport pathway from the near-surface formation region to the deep ocean, especially during the early deglaciation.
-
ArticleEvidence from the Florida Straits for Younger Dryas ocean circulation changes(American Geophysical Union, 2011-02-15) Lynch-Stieglitz, Jean ; Schmidt, Matthew W. ; Curry, William B.The waters passing through the Florida Straits today reflect both the western portion of the wind-driven subtropical gyre and the northward flow of the upper waters which cross the equator, compensating North Atlantic Deep Water export as part of the large-scale Atlantic meridional overturning circulation. It has been postulated from various lines of evidence that the overturning circulation was weaker during the Younger Dryas cold event of the last deglaciation. We show here that the contrast in the oxygen isotopic composition of benthic foraminiferal tests across the Florida Current is reduced during the Younger Dryas. This most likely reflects a decrease in the density gradient across the channel and a decrease in the vertical shear of the Florida Current. This reduced shear is consistent with the postulated reduction in the Atlantic meridional overturning circulation. We find that the onset of this change in density structure and flow at the start of the Younger Dryas is very abrupt, occurring in less than 70 years.
-
ArticleDeglacial variability in the surface return flow of the Atlantic meridional overturning circulation(American Geophysical Union, 2008-03-29) Came, Rosemarie E. ; Oppo, Delia W. ; Curry, William B. ; Lynch-Stieglitz, JeanBenthic foraminiferal Cd/Ca from a Florida Current sediment core documents the history of the northward penetration of southern source waters within the surface return flow of the Atlantic meridional overturning circulation (AMOC). Cd seawater estimates (CdW) indicate that intermediate-depth southern source waters crossed the equator and contributed to the Florida Current during the Bølling-Allerød warm period of the last deglaciation, consistent with evidence of only a modest AMOC reduction compared to today. The CdW estimates also provide the first paleoceanographic evidence of a reduction in the influence of intermediate-depth southern source waters within the Florida Current during the Younger Dryas, a deglacial cold event characterized by a weak North Atlantic AMOC. Our results reveal a close correspondence between the northward penetration of intermediate-depth southern source waters and the influence of North Atlantic Deep Water, suggesting a possible link between intermediate-depth southern source waters and the strength of the Atlantic AMOC.
-
ArticleSeawater cadmium in the Florida Straits over the Holocene and implications for Upper AMOC variability(American Geophysical Union, 2022-04-25) Valley, Shannon G. ; Lynch-Stieglitz, Jean ; Marchitto, Thomas M. ; Oppo, Delia W.Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the global redistribution of heat and precipitation during both abrupt and longer-term climate shifts. Over the next century, AMOC is projected to weaken due to greenhouse gas warming, though projecting its future behavior is dependent on a better understanding of how AMOC changes are forced. Seeking to resolve an apparent contradiction of AMOC trends from paleorecords of the more recent past, we reconstruct seawater cadmium, a nutrient-like tracer, in the Florida Straits over the last ∼8,000 years, with emphasis on the last millennium. The gradual reduction in seawater Cd over the last 8,000 years could be due to a reduction in AMOC, consistent with cooling Northern Hemisphere temperatures and a southward shift of the Intertropical Convergence Zone. However, it is difficult to reconcile this finding with evidence for an increase in geostrophic flow through the Florida Straits over the same time period. We combine data from intermediate water depth sediment cores to extend this record into the Common Era at sufficient resolution to address the broad scale changes of this time period. There is a small decline in the Cd concentration in the Late Little Ice Age relative to the Medieval Climate Anomaly, but this change was much smaller than the changes observed over the Holocene and on the deglaciation. This suggests that any trend in the strength of AMOC over the last millennium must have been very subtle.
-
PreprintMuted change in Atlantic overturning circulation over some glacial-aged Heinrich events( 2013-11-11) Lynch-Stieglitz, Jean ; Schmidt, Matthew W. ; Henry, L. Gene ; Curry, William B. ; Skinner, Luke C. ; Mulitza, Stefan ; Zhang, Rong ; Chang, PingHeinrich events - surges of icebergs into the North Atlantic Ocean - punctuated the last glacial period. The events are associated with millennial-scale cooling in the Northern Hemisphere. Freshwater from the melting icebergs is thought to have interrupted the Atlantic meridional overturning circulation, thus minimizing heat transport into the northern North Atlantic. The northward flow of warm water passes through the Florida Straits and is reflected in the distribution of seawater properties in this region. Here we investigate the northward flow through this region over the past 40,000 years using oxygen isotope measurements of benthic foraminifera from two cores on either side of the Florida Straits, which allow us to estimate water density, which is related to flow via the thermal wind relation. We infer a substantial reduction of flow during Heinrich Event 1 and the Heinrich-like Younger Dryas cooling, but little change during Heinrich Events 2 and 3, which occurred during an especially cold phase of the last glacial period. We speculate that because glacial circulation was already weakened before the onset of Heinrich Events 2 and 3, freshwater forcing had little additional effect. However, low-latitude climate perturbations were observed during all events. We therefore suggest these perturbations may not have been directly caused by changes in heat transport associated with Atlantic overturning circulation as commonly assumed.
-
ArticleAssessing the potential capability of reconstructing glacial Atlantic water masses and AMOC using multiple proxies in CESM(Elsevier, 2020-05-06) Gu, Sifan ; Liu, Zhengyu ; Oppo, Delia W. ; Lynch-Stieglitz, Jean ; Jahn, Alexandra ; Zhang, Jiaxu ; Wu, LixinReconstructing the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM) is essential for understanding glacial-interglacial climate change and the carbon cycle. However, despite many previous studies, uncertainties remain regarding the glacial water mass distributions in the Atlantic and the AMOC intensity. Here we use an isotope enabled ocean model with multiple geotracers (δ 13 C,E Νd,231 Pa/ 230Th,δ 18 Ο and Δ 14 C) and idealized water tracers to study the potential constraints on LGM ocean circulation from multiple proxies. Our model suggests that the glacial Atlantic water mass distribution can be accurately constrained by the air-sea gas exchange signature of water masses (δ13 C AS), but E Nd might overestimate the North Atlantic Deep Water (NADW) percentage in the deep Atlantic probably because of the boundary source of Nd. A sensitivity experiment with an AMOC of similar geometry but much weaker strength suggests that the correct AMOC geometry is more important than the AMOC strength for simulating the observed glacial δ13 C AS and E Nd and distributions. The kinematic tracer 231Pa/230Th is sensitive to AMOC intensity, but the interpretation might be complicated by the AMOC geometry and AABW transport changes during the LGM. δ 18 Ο in the benthic foraminifera (δ 18 Οc) from the Florida Straits provides a consistent measure of the upper ocean boundary current in the model, which potentially provides an unambiguous method to reconstruct glacial AMOC intensity. Finally, we propose that the moderate difference between AMOC intensity at LGM and PD, if any, is caused by the competition of the responses to CO2 forcing and continental ice sheet forcing.
-
ArticleConsistently dated Atlantic sediment cores over the last 40 thousand years(Nature Research, 2019-09-02) Waelbroeck, Claire ; Lougheed, Bryan C. ; Vazquez Riveiros, Natalia ; Missiaen, Lise ; Pedro, Joel ; Dokken, Trond ; Hajdas, Irka ; Wacker, Lukas ; Abbott, Peter ; Dumoulin, Jean-Pascal ; Thil, Francois ; Eynaud, Frederique ; Rossignol, Linda ; Fersi, Wiem ; Albuquerque, Ana Luiza ; Arz, Helge W. ; Austin, William E. N. ; Came, Rosemarie E. ; Carlson, Anders E. ; Collins, James A. ; Dennielou, Bernard ; Desprat, Stéphanie ; Dickson, Alex ; Elliot, Mary ; Farmer, Christa ; Giraudeau, Jacques ; Gottschalk, Julia ; Henderiks, Jorijntje ; Hughen, Konrad A. ; Jung, Simon ; Knutz, Paul ; Lebreiro, Susana ; Lund, David C. ; Lynch-Stieglitz, Jean ; Malaizé, Bruno ; Marchitto, Thomas M. ; Martínez-Méndez, Gema ; Mollenhauer, Gesine ; Naughton, Filipa ; Nave, Silvia ; Nürnberg, Dirk ; Oppo, Delia W. ; Peck, Vicky L. ; Peeters, Frank J. C. ; Penaud, Aurélie ; Portilho-Ramos, Rodrigo da Costa ; Repschläger, Janne ; Roberts, Jenny ; Ruhlemann, Carsten ; Salgueiro, Emilia ; Sanchez Goni, Maria Fernanda ; Schönfeld, Joachim ; Scussolini, Paolo ; Skinner, Luke C. ; Skonieczny, Charlotte ; Thornalley, David J. R. ; Toucanne, Samuel ; Van Rooij, David ; Vidal, Laurence ; Voelker, Antje H. L. ; Wary, Mélanie ; Weldeab, Syee ; Ziegler, MartinRapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies.
-
ArticleMeridional overturning circulation in the South Atlantic at the last glacial maximum(American Geophysical Union, 2006-10-03) Lynch-Stieglitz, Jean ; Curry, William B. ; Oppo, Delia W. ; Ninnemann, Ulysses S. ; Charles, Christopher D. ; Munson, JennaThe geostrophic shear associated with the meridional overturning circulation is reflected in the difference in density between the eastern and western margins of the ocean basin. Here we examine how the density difference across 30°S in the upper 2 km of the Atlantic Ocean (and thus the magnitude of the shear associated with the overturning circulation) has changed between the last glacial maximum and the present. We use oxygen isotope measurements on benthic foraminifera to reconstruct density. Today, the density in upper and intermediate waters along the eastern margin in the South Atlantic is greater than along the western margin, reflecting the vertical shear associated with the northward flow of surface and intermediate waters and the southward flowing North Atlantic Deep Waters below. The greater density along the eastern margin is reflected in the higher δ 18O values for surface sediment benthic foraminifera than those found on the western margin for the upper 2 km. For the last glacial maximum the available data indicate that the eastern margin foraminifera had similar δ 18O to those on the western margin between 1 and 2 km and that the gradient was reversed relative to today with the higher δ 18O values in the western margin benthic foraminifera above 1 km. If this reversal in benthic foraminifera δ 18O gradient reflects a reversal in seawater density gradient, these data are not consistent with a vigorous but shallower overturning cell in which surface waters entering the Atlantic basin are balanced by the southward export of Glacial North Atlantic Intermediate Water.