Lund David C.

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Lund

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David C.

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Florida 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.
Calibration 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, Claire

The 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.
Less remineralized carbon in the intermediate-depth south Atlantic during Heinrich Stadial 1

(American Geophysical Union, 2019-07-24) Lacerra, Matthew ; Lund, David C. ; Gebbie, Geoffrey A. ; Oppo, Delia W. ; Yu, Jimin ; Schmittner, Andreas ; Umling, Natalie E.

The last deglaciation (~20–10 kyr BP) was characterized by a major shift in Earth's climate state, when the global mean surface temperature rose ~4 °C and the concentration of atmospheric CO2 increased ~80 ppmv. Model simulations suggest that the initial 30 ppmv rise in atmospheric CO2 may have been driven by reduced efficiency of the biological pump or enhanced upwelling of carbon‐rich waters from the abyssal ocean. Here we evaluate these hypotheses using benthic foraminiferal B/Ca (a proxy for deep water [CO32−]) from a core collected at 1,100‐m water depth in the Southwest Atlantic. Our results imply that [CO32−] increased by 22 ± 2 μmol/kg early in Heinrich Stadial 1, or a decrease in ΣCO2 of approximately 40 μmol/kg, assuming there were no significant changes in alkalinity. Our data imply that remineralized phosphate declined by approximately 0.3 μmol/kg during Heinrich Stadial 1, equivalent to 40% of the modern remineralized signal at this location. Because tracer inversion results indicate remineralized phosphate at the core site reflects the integrated effect of export production in the sub‐Antarctic, our results imply that biological productivity in the Atlantic sector of the Southern Ocean was reduced early in the deglaciation, contributing to the initial rise in atmospheric CO2.
Late Holocene variability in Florida Current surface density : patterns and possible causes

(American Geophysical Union, 2004-10-05) Lund, David C. ; Curry, William B.

Planktonic foraminiferal δ18O time series from three well-dated, high sedimentation rate cores near the Florida Keys (24.4°N, 83.3°W) exhibit repeated centennial to millennial-scale oscillations during the late Holocene. Isotopic shifts of 0.2–0.3‰ over the past 5200 years represent changes in sea-surface temperature (SST) of 1.0–1.5°C or salinity variability of 1–2 psu. The largest significant isotopic events are centered at approximately 200, 2000, 3200, and prior to 4000 calendar years BP. High Florida Current δ18O during the Little Ice Age (LIA) correlates with published records of high δ18O in the Sargasso Sea and low SST off the coast of west Africa. An interval of generally low δ18O in the Florida Straits from 1800 to 500 years BP is synchronous with the Medieval Warm Period off west Africa but leads low δ18O in the Sargasso Sea by several hundred years. Synchronous cooling across the subtropical gyre during the LIA is difficult to explain using interannual North Atlantic Oscillation patterns but may be consistent with the simulated effects of reduced solar irradiance. At frequencies between 1/1000 and 1/300 years during the Late Holocene, Florida Current δ18O is coherent with a published estimate of 14C production rate. Radiocarbon production seems to lead δ18O at these frequencies, but uncertainty in the phase calculation precludes a clear lead-lag relationship. At frequencies lower than 1/300 years, Florida Current δ18O is coherent and in phase with atmospheric Δ14C. The coherence of Δ14C and δ18O at periods >1000 years implies oceanic circulation may play a role in modulating atmospheric radiocarbon on millennial timescales.
Florida Current surface temperature and salinity variability during the last millennium

(American Geophysical Union, 2006-05-05) Lund, David C. ; Curry, William B.

The salinity and temperature of the Florida Current are key parameters affecting the transport of heat into the North Atlantic, yet little is known about their variability on centennial time scales. Here we report replicated, high-resolution foraminiferal records of Florida Current surface hydrography for the last millennium from two coring sites, Dry Tortugas and the Great Bahama Bank. The oxygen isotopic composition of Florida Current surface water (δ18Ow) near Dry Tortugas increased 0.4‰ during the course of the Little Ice Age (LIA: ~1200-1850 A. D.), equivalent to a salinity increase of 0.8-1.5 psu. On the Great Bahama Bank, where surface waters are influenced by the North Atlantic subtropical gyre, δ18Ow increased by 0.3‰ during the last 200 years. Although a portion (~0.1‰) of this shift may be an artifact of anthropogenically-driven changes in surface water ΣCO2, the remaining δ18Ow signal implies a 0.4 to 1 psu increase in salinity after 200 yr BP. The simplest explanation of the δ18Ow data is southward migration of the Atlantic Hadley circulation during the LIA. Scaling of the δ18Ow records to salinity using the modern low-latitude δ18Ow-S slope produces an unrealistic reversal in the salinity gradient between the two sites. Only if δ18Ow is scaled to salinity using a high-latitude δ18Ow-S slope can the records be reconciled. Changes in atmospheric 14C paralleled shifts in Dry Tortugas δ18Ow, suggesting that variable solar irradiance paced centennialscale ITCZ migration and changes in Florida Current salinity during the last millennium.
Consistently 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, Martin

Rapid 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.
Atlantic circulation and ice sheet influences on upper South Atlantic temperatures during the last deglaciation

(American Geophysical Union, 2019-05-28) Umling, Natalie E. ; Oppo, Delia W. ; Chen, P. ; Yu, Jimin ; Liu, Zhengyu ; Yan, Mi ; Gebbie, Geoffrey A. ; Lund, David C. ; Pietro, Kathryn R. ; Jin, Z. D. ; Huang, Kuo-Fang ; Costa, Karen ; Toledo, Felipe Antonio de Lima

Atlantic Meridional Overturning Circulation (AMOC) disruption during the last deglaciation is hypothesized to have caused large subsurface ocean temperature anomalies, but records from key regions are not available to test this hypothesis, and other possible drivers of warming have not been fully considered. Here, we present the first reliable evidence for subsurface warming in the South Atlantic during Heinrich Stadial 1, confirming the link between large‐scale heat redistribution and AMOC. Warming extends across the Bølling‐Allerød despite predicted cooling at this time, thus spanning intervals of both weak and strong AMOC indicating another forcing mechanism that may have been previously overlooked. Transient model simulations and quasi‐conservative water mass tracers suggest that reduced northward upper ocean heat transport was responsible for the early deglacial (Heinrich Stadial 1) accumulation of heat at our shallower (~1,100 m) site. In contrast, the results suggest that warming at our deeper site (~1,900 m) site was dominated by southward advection of North Atlantic middepth heat anomalies. During the Bølling‐Allerød, the demise of ice sheets resulted in oceanographic changes in the North Atlantic that reduced convective heat loss to the atmosphere, causing subsurface warming that overwhelmed the cooling expected from an AMOC reinvigoration. The data and simulations suggest that rising atmospheric CO2 did not contribute significantly to deglacial subsurface warming at our sites.
230 Th normalization: new insights on an essential tool for quantifying sedimentary fluxes in the modern and quaternary ocean

(John Wiley & Sons, 2020-01-27) Costa, Kassandra M. ; Hayes, Christopher T. ; Anderson, Robert F. ; Pavia, Frank ; Bausch, Alexandra ; Deng, Feifei ; Dutay, Jean-Claude ; Geibert, Walter ; Heinze, Christoph ; Henderson, Gideon M. ; Hillaire‐Marcel, Claude ; Hoffmann, Sharon S. ; Jaccard, Samuel L. ; Jacobel, Allison W. ; Kienast, Stephanie S. ; Kipp, Lauren ; Lerner, Paul ; Lippold, Jörg ; Lund, David C. ; Marcantonio, Franco ; McGee, David ; McManus, Jerry F. ; Mekik, Figen ; Middleton, Jennifer L. ; Missiaen, Lise ; Not, Christelle ; Pichat, Sylvain ; Robinson, Laura F. ; Rowland, George H. ; Roy-Barman, Matthieu ; Tagliabue, Alessandro ; Torfstein, Adi ; Winckler, Gisela ; Zhou, Yuxin

230Th normalization is a valuable paleoceanographic tool for reconstructing high‐resolution sediment fluxes during the late Pleistocene (last ~500,000 years). As its application has expanded to ever more diverse marine environments, the nuances of 230Th systematics, with regard to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of 230Th from across the global ocean at two time slices, the late Holocene (0–5,000 years ago, or 0–5 ka) and the Last Glacial Maximum (18.5–23.5 ka), and investigated the spatial structure of 230Th‐normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79–2.17 g/cm2kyr, 95% confidence) relative to the Holocene (1.48–1.68 g/cm2kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size‐dependent sediment fractionation, and carbonate dissolution on the efficacy of 230Th as a constant flux proxy. Anomalous 230Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that 230Th normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (>1,000 m water depth).
Gulf Stream temperature, salinity and transport during the last millennium

(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2006-02) Lund, David C.

Benthic and planktonic foraminiferal δ18O (δ18Oc) from a suite of well-dated, high-resolution cores spanning the depth and width of the Straits of Florida reveal significant changes in Gulf Stream cross-current density gradient during the last millennium. These data imply that Gulf Stream transport during the Little Ice Age (LIA: 1200-1850 A.D.) was 2-3 Sv lower than today. The timing of reduced flow is consistent with cold conditions in Northern Hemisphere paleoclimate archives, implicating Gulf Stream heat transport in centennial-scale climate variability of the last 1,000 years. The pattern of flow anomalies with depth suggests reduced LIA transport was due to weaker subtropical gyre wind stress curl. The oxygen isotopic composition of Florida Current surface water (δ18O w) near Dry Tortugas increased 0.4% during the course of the Little Ice Age (LIA: ~1200-1850 A.D.), equivalent to a salinity increase of 0.8-1.5 psu. On the Great Bahama Bank, where smface waters are influenced by the North Atlantic subtropical gyre, δ18Ow increased by 0.3% during the last 200 years. Although a portion (~0.1%) of this shift may be an artifact of anthropogenically-driven changes in surface water ΣCO2, the remaining δl8Ow signal implies a 0.4 to 1 psu increase in salinity after 200 yr BP. The simplest explanation of the δ18Ow data is southward migration of the Atlantic Hadley circulation during the LIA. Scaling of the δ18Ow records to salinity using the modern low-latitude δ18Ow-S slope produces an unrealistic reversal in the salinity gradient between the two sites. Only if C180 w is scaled to salinity using a high-latitude δ18Ow-S slope can the records be reconciled. Changes in atmospheric 14C paralleled shifts in Dry Tortugas δ18Ow , suggesting that variable solar irradiance paced centennial-scale Hadley cell migration and changes in Florida Current salinity during the last millennium.
Brazil margin stable isotope profiles for the last glacial cycle: implications for watermass geometry and oceanic carbon storage

(American Geophysical Union, 2024-01-12) Shub, Alec B. ; Lund, David C. ; Oppo, Delia W. ; Garity, Monica L.

Vertical profiles of benthic foraminiferal oxygen and carbon isotopes (δ18O and δ13C) imply the volume of southern source water (SSW) in the Atlantic basin expanded during the Last Glacial Maximum. Shoaling of the boundary between SSW and northern source water (NSW) may reduce mixing between the two watermasses, thereby isolating SSW and enhancing its ability to store carbon during glacial intervals. Here we test this hypothesis using profiles of δ18O and δ13C from the Brazil Margin spanning the last glacial cycle (0–150 ka). Shoaling of the SSW-NSW boundary occurred during Marine Isotope Stage (MIS) 2, 4, and 6, consistent with expansion of SSW and greater carbon sequestration in the abyss. But the watermass boundary also shoaled during MIS 5e, when atmospheric CO2 levels were comparable to MIS 1. Additionally, we find there was little change in watermass structure across the MIS 5e-d transition, the first major decline in CO2 of the last glacial cycle. Thus, the overall pattern in glacial-interglacial geometry is inconsistent with watermass mixing acting as a primary control on atmospheric pCO2. We also find that δ13C values for MIS 5e are systematically lower than MIS 1, with the largest difference (∼1‰) occurring in the upper water column. Low δ13C during MIS 5e was most likely due to a long-term imbalance in weathering and deposition of calcium carbonate or input of 13C-depleted carbon from a reservoir external to the ocean-atmosphere system.

Lund David C.

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