James L Cullen

Palaeo-oceanography: Deepwater variability in the Holocene epoch.

The conversion of surface water to deep water in the North Atlantic results in the release of heat from the ocean to the atmosphere, which may have amplified millennial-scale climate variability during glacial times and could even have contributed to the past 11,700 years of relatively mild climate (known as the Holocene epoch). Here we investigate changes in the carbon-isotope composition of benthic foraminifera throughout the Holocene and find that deep-water production varied on a centennial–millennial timescale. These variations may be linked to surface and atmospheric events that hint at a contribution to climate change over this period.

Persistent suborbital climate variability in marine isotope stage 5 and termination II

New surface water records from two high sedimentation rate sites, located in the western subtropical North Atlantic near the axis of the Gulf Stream, provide clear evidence of suborbital climate variations through marine isotope stage (MIS) 5 persisting even into the warm peak of the interglaciation (substage 5e). We found that the amplitude of suborbital climate oscillations did not vary significantly for the whole of MIS 5, implying that ice volume has little or no influence on the amplitude of suborbital climate variability in this region. Although some records suggest that longer suborbital variations (4–10 kyr) during MIS 5 are linked to deepwater changes, none of the existing records is of sufficient resolution to assess if a linkage occurred for oscillations shorter than 4 kyr. However, when examined in conjunction with published data from the Norwegian Sea, new evidence from the subpolar North Atlantic suggests that coupled surface-deepwater oscillations occurred during the penultimate deglaciation. This supports the hypothesis that during glacial and deglacial times, ocean-ice interactions and deepwater variability amplify suborbital climate change at higher latitudes. We suggest that during the penultimate deglaciation the North Atlantic deepwater source varied between Nordic Sea and open North Atlantic locations, in parallel with surface temperature oscillations.

North Atlantic intermediate to deep water circulation and chemical stratification during the past 1 Myr

Benthic foraminiferal carbon isotope records from a suite of drill sites in the North Atlantic are used to trace variations in the relative strengths of Lower North Atlantic Deep Water (LNADW), Upper North Atlantic Deep Water (UNADW), and Southern Ocean Water (SOW) over the past 1 Myr. During glacial intervals, significant increases in intermediate-to-deep δ13C gradients (commonly reaching >1.2‰) are consistent with changes in deep water circulation and associated chemical stratification. Bathymetric δ13C gradients covary with benthic foraminiferal δ18O and covary inversely with Vostok CO2, in agreement with chemical stratification as a driver of atmospheric CO2 changes. Three deep circulation indices based on δ13C show a phasing similar to North Atlantic sea surface temperatures, consistent with a Northern Hemisphere control of NADW/SOW variations. However, lags in the precession band indicate that factors other than deep water circulation control ice volume variations at least in this band.

Marine Isotope Stage 11 (Mis 11): Analog for Holocene and Future Climate?

Pleistocene interglacials are often considered to be possible geological analogs for the climatic development of the Holocene epoch. Marine isotope stage 11 (MIS 11), a prominent interglacial 400 ky ago, is of particular interest because of the similarity between orbitally driven insolation variations then and now. We have examined the record of climatic conditions during MIS 11 at two locations on rapidly accumulating sediment drifts in the North Atlantic, and made a comparison with global records in order to assess the duration, stability, and amplitude of the interglacial. Deep-sea cores from ODP Sites 980 and 983 have sedimentation rates in excess of 10 cm/kyr, and have been sampled at 2-3 cm intervals, yielding century-scale resolution of millennial-scale variability. We used stable isotopes of oxygen in foraminifera to assess climatic and hydrographic conditions at the sea surface and in the deep ocean. Different age models were evaluated, including one tuned to orbital insolation variations and one based on a constant accumulation model. These chronologies indicate that the relatively ice-free portion of MIS 11 lasted longer than other peak interglacials. Sea surface warmth in the subpolar North Atlantic lasted even longer, a minimum of 30 kyr. Throughout this interval, oxygen isotope ratios in Neogloboquadrina pachyderma (dextral), a proxy for sea-surface temperature (SST), did not vary more than 0.25 per mil, or ∼1°C, from the long-term mean. This is in strong contrast to the large temperature oscillations in the subsequent glaciation, MIS 10. During both the interglacial and glacial, a gradient in planktonic oxygen isotope ratios was maintained between the two sites, counter to the modern salinity-driven gradient in seawater oxygen isotope ratios, and therefore consistent with a persistent similar N-S temperature gradient. Oxygen isotope ratios recorded during MIS 11 in both planktonic and benthic foraminifera are similar to values that characterize the Holocene. Thus ice-volume (sea-level), ocean temperature, local salinity, and the isotopic composition of ice sheets were similar, in sum, to today. Any departure from the modern values in one of these climate components would have had to be compensated by some combination of the others. We conclude that the elapsed portion of the Holocene has been similar to MIS 11 in mean climate state and degree of stability, without nearly approaching its duration. Both the forcing and response of climate during MIS 11 appear to be appropriate analogs for the natural development of recent and future climate.