Ulysses S. Ninnemann

Surface and subsurface seawater temperature reconstruction using Mg/Ca microanalysis of planktonic foraminifera Globigerinoides ruber, Globigerinoides sacculifer, and Pulleniatina obliquiloculata

Laser-ablation inductively coupled plasma-mass spectrometry microanalyses of Mg/Ca across individual final chambers of three planktonic foraminifera species, Globigerinoides ruber, G. sacculifer, and Pulleniatina obliquiloculata, reveal significant interspecies differences in test Mg concentrations. Whereas these three species have similar Mg/Ca values at low sea surface temperatures (similar to 22 degrees C), they diverge markedly at high sea surface temperatures (similar to 29 degrees C). Explanations for these differences in species Mg/Ca values based on detailed comparison of species intratest Mg/Ca distributions suggest that compositional variability within tests cannot account for the observed deviation of species Mg/Ca values in warm-water equatorial regions. Multiple regression modeling and delta O-18 analysis of Globigerinoides sacculifer tests indicate that interspecies differences in Mg/Ca values result from different depth habitats. The average Mg/Ca values of G. ruber final chambers reflect the temperature of the surface mixed layer (0-25 m), whereas those of G. sacculifer and Pulleniatina obliquiloculata correlate best with subsurface temperatures at 50-75 m and 100-125 m water depths, respectively. Mg/Ca calibration to the temperatures at these depths reveals a similar temperature control on Mg test composition in all species. Combining our results with Mg/Ca values from published culturing experiments, we derive a generalized equation for the effect of temperature and seawater salinity on foraminiferal Mg/Ca. We also show that the Mg/Ca composition of specific calcite layers within foraminiferal tests, including the low-Mg/Ca layers of Globigerinoides ruber and G. sacculifer and the cortex layer of Pulleniatina obliquiloculata, correlates with seawater temperature and can be used as an additional proxy for seawater temperature.

Rapid Reductions in North Atlantic Deep Water During the Peak of the Last Interglacial Period

Limited Stability Deep ocean circulation is thought to be stable during warm, interglacial periods. Galaasen et al. (p. 1129, published online 20 February) constructed a highly resolved record of North Atlantic Deep Water production during the last interglacial period, around 128,000 to 116,000 years ago. The findings reveal large, centennial-scale reductions—in contrast to the prevailing paradigm. These changes occurred in an ocean warmer than that of today, but in a temperature regime similar to that expected because of global warming, raising the possibility that future ocean circulation, regional climate, and CO2 sequestration pathways could be impacted. Deep ocean circulation was less stable during the last interglacial periods than previously supposed. Deep ocean circulation has been considered relatively stable during interglacial periods, yet little is known about its behavior on submillennial time scales. Using a subcentennially resolved epibenthic foraminiferal δ13C record, we show that the influence of North Atlantic Deep Water (NADW) was strong at the onset of the last interglacial period and was then interrupted by several prominent centennial-scale reductions. These NADW transients occurred during periods of increased ice rafting and southward expansions of polar water influence, suggesting that a buoyancy threshold for convective instability was triggered by freshwater and circum-Arctic cryosphere changes. The deep Atlantic chemical changes were similar in magnitude to those associated with glaciations, implying that the canonical view of a relatively stable interglacial circulation may not hold for conditions warmer and fresher than at present.

Rapid switches in subpolar North Atlantic hydrography and climate during the Last Interglacial (MIS 5e)

plateauing just below early MIS 5e values. A planktonic d 18 O minimum during the cooling event indicates that marked freshening of the surface waters accompanied the cooling. We suggest that switches in the subpolar gyre hydrography occurred during a warmer climate, involving regional changes in freshwater fluxes/balance and East Greenland Current influence in the study area. The nature of these hydrographic transitions suggests that they are most likely related to large-scale circulation dynamics, potentially amplified by GIS meltwater influences.

Drilling reveals climatic consequences of Tasmanian Gateway Opening

One of the great stories of geoscience is how Gondwana broke up and the other southern continents drifted northward from Antarctica, which led to major changes in global climate. The recent drilling of Ocean Drilling Project (ODP) Leg 189 addressed in detail what happened as Australia drifted away from Antarctica and the Tasmanian Gateway opened. The drifting contributed to the change in global climate, from relatively warm early Cenozoic “greenhouse” conditions to late Cenozoic “icehouse” conditions. It isolated Antarctica from warm gyral surface currents from the north and provided the critical deepwater conduits that eventually led to ocean conveyor circulation between the Atlantic and Pacific Oceans.

Large δ13C Gradients in the Preindustrial North Atlantic Revealed

Lost Details Changes in ocean circulation are commonly inferred by differences between the distribution of carbon isotopes in the past and now, but making such comparisons neglects the fact that modern fossil fuel burning has modified the carbon isotopic composition of the ocean. This in turn obscures details about recent mass movement of water. Olsen and Ninnemann (p. 658) correct for this effect in the North Atlantic and show that the natural distribution of carbon isotopes has more detail and is clearly related to water mass distributions. The results change some important ideas about glacial-interglacial ocean variations within the context of modern climate variability. The preanthropogenic distribution of carbon isotopes in the North Atlantic provides a correct baseline for climate studies. The carbon isotopic composition (13C/12C, expressed as δ13C) of fossil foraminifera is the primary tracer used to infer changes in past ocean ventilation, and its variations are interpreted by using the modern oceanic δ13C distribution as a framework. However, the present ocean δ13C distribution is strongly overprinted by isotopically light anthropogenic carbon dioxide. A correction for this oceanic C-13 Suess effect in the North Atlantic (NA) shows that the pristine NA δ13C distribution has a richer and more detailed structure that is more clearly related to water mass distributions. Our results revise some fundamental perceptions regarding glacial-interglacial ocean δ13C differences and allow paleo-δ13C variations to be understood within the context of modern climate variability.

Increased ventilation of Antarctic deep water during the warm mid-Pliocene

The mid-Pliocene warm period is a recent warm geological period that shares similarities with predictions of future climate. It is generally held the mid-Pliocene Atlantic Meridional Overturning Circulation must have been stronger, to explain a weak Atlantic meridional δ13C gradient and large northern high-latitude warming. However, climate models do not simulate such stronger Atlantic Meridional Overturning Circulation, when forced with mid-Pliocene boundary conditions. Proxy reconstructions allow for an alternative scenario that the weak δ13C gradient can be explained by increased ventilation and reduced stratification in the Southern Ocean. Here this alternative scenario is supported by simulations with the Norwegian Earth System Model (NorESM-L), which simulate an intensified and slightly poleward shifted wind field off Antarctica, giving enhanced ventilation and reduced stratification in the Southern Ocean. Our findings challenge the prevailing theory and show how increased Southern Ocean ventilation can reconcile existing model-data discrepancies about Atlantic Meridional Overturning Circulation while explaining fundamental ocean features.

Paleo-export production, terrigenous flux and sea surface temperatures around Tasmania: Implications for glacial/interglacial changes in the Subtropical Convergence zone

The Tasmanian Gateway, focus of ODP Leg 189, is a key oceanographic area within the Southern Ocean. Our investigations concentrate on the last -500,000 years of sedimentation at Sites 1168 (western Tasmanian margin), 1170 and 1171 (Tasman Rise), and 1172 (East Tasman Plateau). A suite of geochemical proxy data reflecting paleo-export production, terrigenous flux, and sea surface temperature, constrain temporal and spatial variations in surface water masses and oceanographic frontal systems over these sites. Interglacial periods were commonly of low productivity and less affected by terrigenous matter supply, suggesting that the position of the Subtropical Convergence remained south of Tasman Rise. Only during early MIS 11 and MIS 9 over the southern Tasman Rise, and during MIS 7 over the northern Tasman Rise, did enhanced marine productivity, combined with an enhanced terrigenous flux, indicate waxing influence of subantarctic waters. During glacial MIS 2, marine productivity and terrigenous flux increased significantly at Sites 1168, 1170, and 1171 implying that the Subtropical Convergence moved northward to ∼42°S west of Tasmania. East of Tasmania, the presence of the East Australian Current caused the Subtropical Convergence to remain south of East Tasman Plateau. Glacial MIS 6 appears to have been different from MIS 2. The Subtropical Convergence stayed north of East Tasman Plateau, but clearly south of Site 1168 on the western Tasmanian margin. Strongly enhanced marine productivity and terrigenous flux during MIS 10 and 12 at Sites 1168, 1170, and 1172 suggest the dominant influence of subantarctic waters and the position of the Subtropical Convergence north of East Tasman Plateau. At South Tasman Rise, in contrast, the reduced terrigenous flux implies that Site 1171 moved outside the belt of westerly winds. Marine productivity ceased at that time mainly due to iron limitation.

A global estimate of the full oceanic 13C Suess effect since the preindustrial

We present the first estimate of the full global ocean 13C Suess effect since preindustrial times, based on observations. This has been derived by first using the method of Olsen and Ninnemann (2010) to calculate 13C Suess effect estimates on sections spanning the world ocean, which were next mapped on a global 1° × 1° grid. We find a strong 13C Suess effect in the upper 1000 m of all basins, with strongest decrease in the subtropical gyres of the Northern Hemisphere, where δ13C of dissolved inorganic carbon has decreased by more than 0.8‰ since the industrial revolution. At greater depths, a significant 13C Suess effect can only be detected in the northern parts of the North Atlantic Ocean. The relationship between the 13C Suess effect and the concentration of anthropogenic carbon varies strongly between water masses, reflecting the degree to which source waters are equilibrated with the atmospheric 13C Suess effect before sinking. Finally, we estimate a global ocean inventory of anthropogenic CO2 of 92 ± 46 Gt C. This provides an estimate that is almost independent of and consistent, within the uncertainties, with previous estimates.

Holocene multidecadal- to millennial-scale variations in Iceland-Scotland overflow and their relationship to climate

The Nordic Seas overflows are an important part of the Atlantic thermohaline circulation. While there is growing evidence that the overflow of dense water changed on orbital time scales during the Holocene, less is known about the variability on shorter time scales beyond the instrumental record. Here we reconstruct the relative changes in flow strength of Iceland-Scotland Overflow Water (ISOW), the eastern branch of the overflows, on multidecadal-millennial time scales. The reconstruction is based on mean sortable silt ( SS¯) from a sediment core on the Gardar Drift (60°19′N, 23°58′W, 2081 m). Our SS¯ record reveals that the main variance in ISOW vigor occurred on millennial time scales (1–2 kyr) with particularly prominent fluctuations after 8 kyr. Superimposed on the millennial variability, there were multidecadal-centennial flow speed fluctuations during the early Holocene (10–9 kyr) and one prominent minimum at 0.9 kyr. We find a broad agreement between reconstructed ISOW and regional North Atlantic climate, where a strong (weak) ISOW is generally associated with warm (cold) climate. We further identify the possible contribution of anomalous heat and freshwater forcing, respectively, related to reconstructed overflow variability. We infer that ocean poleward heat transport can explain the relationship between regional climate and ISOW during the middle to late Holocene, whereas freshwater input provides a possible explanation for the reduced overflow during early Holocene (8–10 kyr).

Pacific‐Atlantic Circumpolar Deep Water coupling during the last 500 ka

Investigating the interbasin deepwater exchange between the Pacific and Atlantic Oceans over glacial-interglacial climate cycles is important for understanding circum-Antarctic Southern Ocean circulation changes and their impact on the global Meridional Overturning Circulation. We use benthic foraminiferal δ13C records from the southern East Pacific Rise to characterize the δ13C composition of Circumpolar Deep Water in the South Pacific, prior to its transit through the Drake Passage into the South Atlantic. A comparison with published South Atlantic deepwater records from the northern Cape Basin suggests a continuous water mass exchange throughout the past 500 ka. Almost identical glacial-interglacial δ13C variations imply a common deepwater evolution in both basins suggesting persistent Circumpolar Deep Water exchange and homogenization. By contrast, deeper abyssal waters occupying the more southern Cape Basin and the southernmost South Atlantic have lower δ13C values during most, but not all, stadial periods. We conclude that these values represent the influence of a more southern water mass, perhaps Antarctic Bottom Water (AABW). During many interglacials and some glacial periods, the gradient between Circumpolar Deep Water and the deeper southern Cape Basin bottom water disappears suggesting either no presence of AABW or indistinguishable δ13C values of both water masses.