Rosemarie E. Came

Coupling of surface temperatures and atmospheric CO2 concentrations during the Palaeozoic era.

Atmospheric carbon dioxide concentrations seem to have been several times modern levels during much of the Palaeozoic era (543–248 million years ago), but decreased during the Carboniferous period to concentrations similar to that of today. Given that carbon dioxide is a greenhouse gas, it has been proposed that surface temperatures were significantly higher during the earlier portions of the Palaeozoic era. A reconstruction of tropical sea surface temperatures based on the δ18O of carbonate fossils indicates, however, that the magnitude of temperature variability throughout this period was small, suggesting that global climate may be independent of variations in atmospheric carbon dioxide concentration. Here we present estimates of sea surface temperatures that were obtained from fossil brachiopod and mollusc shells using the ‘carbonate clumped isotope’ method—an approach that, unlike the δ18O method, does not require independent estimates of the isotopic composition of the Palaeozoic ocean. Our results indicate that tropical sea surface temperatures were significantly higher than today during the Early Silurian period (443–423 Myr ago), when carbon dioxide concentrations are thought to have been relatively high, and were broadly similar to today during the Late Carboniferous period (314–300 Myr ago), when carbon dioxide concentrations are thought to have been similar to the present-day value. Our results are consistent with the proposal that increased atmospheric carbon dioxide concentrations drive or amplify increased global temperatures.

Amplitude and timing of temperature and salinity variability in the subpolar North Atlantic over the past 10 k.y.

Paired planktic foraminiferal δ 18 O and Mg/Ca data reveal trends of increasing temperatures (~3 °C) and salinities in the subpolar North Atlantic over the course of the Holocene, which were punctuated by abrupt events. The trends likely refl ect an insolation-forced northward retreat of the boundary between polar and North Atlantic subsurface waters. The superimposed variability does not appear to be periodic, but tends to recur within a broad millennial band. The records provide convincing evidence of open-ocean cooling (nearly 2 °C) and freshening during the 8.2 ka event, and suggest similar conditions at 9.3 ka. However, the two largest temperature oscillations in our record (~2 °C) occurred during the past 4 k.y., suggesting a recent increase in temperature variability relative to the mid-Holocene, perhaps in response to neoglaciation, which began at about this time.

Subtropical Atlantic salinity variability and Atlantic meridional circulation during the last deglaciation

During the last deglaciation (ca. 21–10 ka), freshening of the North Atlantic surface likely caused reductions in Atlantic meridional overturning circulation (AMOC); the mechanisms related to AMOC recovery remain poorly understood. Here we present three new deglacial surface temperature and δ18Oseawater (δ18Osw) reconstructions from the western subtropical North and South Atlantic. Similarities to tropical Caribbean and western Atlantic δ18Osw records suggest that a salty surface water mass accumulated in the western Atlantic from 27°S to 33°N during periods of reduced AMOC. However, δ18Osw decreases led deep AMOC resumption by hundreds of years. We suggest that the northward export of salt previously trapped in the western Atlantic resulted in the early establishment of a shallow overturning circulation that eventually culminated in deep AMOC resumption, implying that AMOC may constitute a self-limiting system.

Atlantic Ocean circulation during the Younger Dryas: Insights from a new Cd/Ca record from the western subtropical South Atlantic

Author Posting.

Deglacial variability in the surface return flow of the Atlantic meridional overturning circulation

[1] Benthic 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 Bolling-Allerod warm period of the last deglaciation, consistent with evidence of only a modest AMOC reduction compared to today. The CdWestimates 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.

Regional climate variability in the western subtropical North Atlantic during the past two millennia

[1] Western subtropical North Atlantic oceanic and atmospheric circulations connect tropical and subpolar climates. Variations in these circulations can generate regional climate anomalies that are not reflected in Northern Hemisphere averages. Assessing the significance of anthropogenic climate change at regional scales requires proxy records that allow recent trends to be interpreted in the context of long‐term regional variability. We present reconstructions of Gulf Stream sea surface temperature (SST) and hydrographic variability during the past two millennia based on the magnesium/calcium ratio and oxygen isotopic composition of planktic foraminifera preserved in two western subtropical North Atlantic sediment cores. Reconstructed SST suggests low‐frequency variability of ∼1°C during an interval that includes the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). A warm interval near 1250 A.D. is distinct from regional and hemispheric temperature, possibly reflecting regional variations in ocean‐atmosphere heat flux associated with changes in atmospheric circulation (e.g., the North Atlantic Oscillation) or the Atlantic Meridional Overturning Circulation. Seawater d 18 O, which is marked by a fresher MCA and a more saline LIA, covaries with meridional migrations of the Atlantic Intertropical Convergence Zone. The northward advection of tropical salinity anomalies by mean surface currents provides a plausible mechanism linking Carolina Slope and tropical Atlantic hydrology.

North Atlantic Intermediate Depth Variability During the Younger Dryas: Evidence from Benthic Foraminiferal Mg/Ca and the Gfdl R30 Coupled Climate Model

The transition from the most recent glaciation to the Holocene interglacial was punctuated by several millennial scale climate oscillations [Bond et al., 1993; Dansgaard et al., 1993; 1992]. The most dramatic of these oscillations was the Younger Dryas, which was a near-glacial cold event in the North Atlantic that lasted from ∼13,000 to ∼11,500 years before present. Many paleoclimate records from the circumNorth Atlantic reveal a significant cooling during the Younger Dryas, including oxygen isotopes in Greenland ice cores [Dansgaard, 1984] and the faunal assemblages of planktic foraminifera [Ruddiman and McIntyre, 1981]. More recently, however, the widespread nature of the event has been revealed in paleoclimate records from around the globe, North Atlantic Intermediate Depth Variability During the Younger Dryas: Evidence From Benthic Foraminiferal Mg/Ca and the GFDL R30 Coupled Climate Model