Kathryn A Venz

A 1.0 Myr Record of Glacial North Atlantic Intermediate Water Variability from ODP Site 982 in the Northeast Atlantic

Changes in the intermediate water structure of the North Atlantic were reconstructed using benthic foraminiferal δ13C at Ocean Drilling Program (ODP) site 982 for the past 1.0 Myr. During most terminations of the late Pleistocene, melting of icebergs and low-salinity surface waters caused production of Glacial North Atlantic Intermediate Water to cease, resulting in decreased ventilation of the middepth North Atlantic. Poor ventilation of intermediate water masses lasted well into some interglacial stages until upper North Atlantic Deep Water (NADW) production resumed under full interglacial conditions. The magnitude of benthic δ13C minima and ice-rafted debris maxima at terminations at site 982 generally match the degree of glacial suppression of NADW inferred from site 607. These processes may be related and controlled by the spatial and seasonal extent of sea ice cover during glaciations in the Nordic Seas.

New evidence for changes in Plio–Pleistocene deep water circulation from Southern Ocean ODP Leg 177 Site 1090

Abstract Changes in Atlantic deep water circulation were reconstructed by comparing the benthic foraminiferal δ13C record at ODP Site 1090 in the South Atlantic with similar records from the North Atlantic (Sites 982, 607, 925, 929) and deep Pacific (Site 849) oceans. Important deep water circulation changes occurred in the early Pleistocene at 1.55 Myr and during the Mid-Pleistocene Transition at 0.9 Myr. At 1.55 Myr, glacial δ13C values in the Southern Ocean became significantly lower than those in the deep Pacific, establishing a pattern that persisted throughout the late Pleistocene. We propose that the lowering of δ13C values of Southern Component Water (SCW) at this time resulted from expansion of sea ice and reduced ventilation of deep water during glacial periods after marine isotope stage 52. Accompanying this change in Southern Ocean deep water circulation was enhanced interhemispheric coupling between the North and South Atlantic after 1.55 Myr. At ∼0.9 Myr, the magnitude of glacial-to-interglacial variability in δ13C increased and shifted to a longer frequency (100 kyr) along with oceanic δ18O (ice volume). Calculation of percent Northern Component Water (NCW) using Site 1090 as the SCW end member yielded 20–30% less reduction of NCW during glacial periods of the late Pleistocene. Also, a trend toward reduced glacial suppression of NCW during the past 400 kyr is not evident. The apparent decoupling of ice volume and deep water circulation reported previously may be an artifact of using a Pacific, rather than a Southern Ocean, carbon isotopic record to calculate past mixing ratios of NCW and SCW.

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.

The mid-Brunhes transition in ODP sites 1089 and 1090 (subantarctic South Atlantic)

We studied cores from ODP sites 1089 and 1090 in the subantarctic South Atlantic to reconstruct paleoceanographic changes during the mid-Brunhes in the context of climate evolution of the Pleistocene. The mid-Brunhes event is marked by an abrupt shift toward lower δ 18 O values during interglacial stages beginning with MIS 11, consistent with Jansen et al. [1986] who first proposed a mid-Brunhes transition to more humid, interglacial conditions in the southern hemisphere. In addition, we identified the mid-Brunhes dissolution cycle as part of a long-period oscillation that is expressed in dissolution indices and planktic δ 13 C, which reach maximum values during interglacial stages 13 and 11. Taking advantage of the high sedimentation rates at site 1089 (15 cm/kyr), we enumerate the sequence of events that occurred during Termination V and MIS 11 and speculate about their cause(s). A comparison between site 1089 and the Vostok ice core suggests that peak conditions of stage 11 are accurately captured in the ice core record, and that temperatures in the high-latitude southern hemisphere and global pCO 2 levels during stage 11 were similar to the Holocene. Furthermore, a remarkable correlation between Vostok pCO 2 and % foraminiferal fragmentation at site 1089 suggests a strong coupling of the marine carbonate system and atmospheric pCO 2 during the mid-Brunhes. Although stage 11 and the Holocene share some similarities (e.g., orbital configuration, pCO 2 , etc.), caution is advised in using stage 11 as an analog for the Holocene because the maximum in dissolution and δ 13 C during the mid-Brunhes indicate that the marine carbonate-carbon cycle was fundamentally different than today.

Determination of Antarctic ice sheet stability over the last 500 ka through a study of iceberg-rafted debris

We have analyzed ice‐rafted debris (IRD) from the South Atlantic Ocean (∼43°S, 9°E) in order to investigate Antarctic Ice Sheet history during the late Pleistocene; the cores examined for this study include piston core TN057‐6‐PC4 and Ocean Drilling Program Leg 177 drill core Site 1090 (177‐1090). Over the last 500 ka at this distal location, IRD arrived during both glacials and interglacials. IRD is present even during warmer intervals, is greatest during colder intervals, and is absent only during terminations and a few other brief intervals. Four different methods are used to normalize the IRD counts, which are then compared to support our interpretation. Several other high‐quality climate proxies from this location also aid our interpretations. We conclude that sea surface temperatures are the primary control on the delivery of IRD to this site. During cold times more icebergs survived to reach this distal location. During warm times only a few of the largest icebergs could travel this far. Garnets found in these sediments suggest a likely East Antarctic origin for the IRD; the presence of garnets even during warm intervals further strongly supports that the iceberg source must be the East Antarctic Ice Sheet (EAIS). Therefore, the EAIS must have continued to reach the ocean at least in some part of its margin throughout the last 500 ka. On the other hand, we cannot specifically trace any IRD to the West Antarctic Ice Sheet (WAIS), so WAIS persistence cannot be tested. A particular radiolarian, identified as Dictyocoryne profunda (Ehrenberg) (sensu Boltovskoy (1998)), shows up in the examined size fraction generally only during warm phases. We suggest that D. profunda is a sensitive indicator of warm water temperatures and that it deserves further study.