Age determination of sediment cores of the southeastern Weddell Sea, Antarctica

On the continental margin of the southeastern Weddell Sea, Antarctica, several channel-ridge systems can be traced on the eastern side of the Crary Fan. Swath mapping of the bathymetry reveals three southwest-northeast trending ridges up to 300 m high with channels on their southeastern side. The structures occur on a terrace of the continental slope in water depths of 2000 - 3300 m. We carried out sedimentological studies on cores from three sites. Two of the studied cores are from ridges, one is from the northwestern part of the terrace. The stratigraphy of the recovered sediments is based on accelerator mass spectrometer 14C determinations, stable oxygen and carbon isotopes analyses and paleomagnetic measurements. The sediments represent a period from the last glacial maximum (LGM) to recent time. They are composed predominantly of terrigenous components. We distinguish four different sedimentary facies and assign them to processes controlling sedimentation. Microlaminated muds and cross-stratified coarse-silty sediments originated from contour currents. Bioturbated sediments reflect the increasing influence of hemipelagic sedimentation. Structureless sediments with high contents of ice-rafted debris characterize slumps. The inferred contour currents shaping the continental slope during the LGM were canalized within the channels and supplied microlaminated mud to the western sedimentary ridges due to deflection to the left induced by the Coriolis force. The lamination of the sediments is attributed to seasonal variations of current velocities. The thermohaline bottom currents were directed to the northeast and hence opposite to the Weddell Gyre. Cross-stratified coarse-silty contourites on the ridges are intercalated with the muds and indicate spillover of faster thermohaline flows. Average sedimentation rates on the terrace of the continental slope were unusually high (250 cm/ka) during the LGM, indicating active growth phases of the Crary Fan during glacial intervals. A substantial environmental change at 19.5 - 20 ka is documented in the sediments by a gradual change from lamination to bioturbation. During the recent interglacial, bioturbated sediments were deposited in all parts of the terrace. Because of a reduction of the contour current velocities (4-7 cm/s), the water masses of the Weddell Gyre, supplying fine-grained sediments from northeast, gain a greater influence on sedimentation on the continental slope. Higher percentages of microfossils indicate enhanced biogenic productivity. Increased iceberg activity is documented by greater amounts of ice-rafted debris. The interglacial sedimentation rates decrease to a few cm/ka and indicate that the Crary Fan became relatively sediment-starved during interglacial intervals.

Tropical climate and vegetation simulations during the Heinrich event 1 using an Earth System Model of Intermediate Complexity (EMIC) - the University of Victoria Earth System-Climate Model (UVic ESCM)

Abrupt climate changes from 18 to 15 thousand years before present (kyr BP) associated with Heinrich Event 1 (HE1) had a strong impact on vegetation patterns not only at high latitudes of the Northern Hemisphere, but also in the tropical regions around the Atlantic Ocean. To gain a better understanding of the linkage between high and low latitudes, we used the University of Victoria (UVic) Earth System-Climate Model (ESCM) with dynamical vegetation and land surface components to simulate four scenarios of climate-vegetation interaction: the pre-industrial era, the Last Glacial Maximum (LGM), and a Heinrich-like event with two different climate backgrounds (interglacial and glacial). We calculated mega-biomes from the plant-functional types (PFTs) generated by the model to allow for a direct comparison between model results and palynological vegetation reconstructions. Our calculated mega-biomes for the pre-industrial period and the LGM corresponded well with biome reconstructions of the modern and LGM time slices, respectively, except that our pre-industrial simulation predicted the dominance of grassland in southern Europe and our LGM simulation resulted in more forest cover in tropical and sub-tropical South America. The HE1-like simulation with a glacial climate background produced sea-surface temperature patterns and enhanced inter-hemispheric thermal gradients in accordance with the "bipolar seesaw" hypothesis. We found that the cooling of the Northern Hemisphere caused a southward shift of those PFTs that are indicative of an increased desertification and a retreat of broadleaf forests in West Africa and northern South America. The mega-biomes from our HE1 simulation agreed well with paleovegetation data from tropical Africa and northern South America. Thus, according to our model-data comparison, the reconstructed vegetation changes for the tropical regions around the Atlantic Ocean were physically consistent with the remote effects of a Heinrich event under a glacial climate background.

(Table 1) Age determination of western North Atlantic sediment cores

Foraminiferal abundance, 14C ventilation ages, and stable isotope ratios in cores from high deposition rate locations in the western subtropical North Atlantic are used to infer changes in ocean and climate during the Younger Dryas (YD) and Last Glacial Maximum (LGM). The d18O of the surface dwelling planktonic foram Globigerinoides ruber records the present-day decrease in surface temperature (SST) of ~4°C from Gulf Stream waters to the northeastern Bermuda Rise. If during the LGM the modern d18O/salinity relationship was maintained, this SST contrast was reduced to 2°C. With LGM to interglacial d18O changes of at least 2.2 per mil, SSTs in the western subtropical gyre may have been as much as 5°C colder. Above ~2.3 km, glacial d13C was higher than today, consistent with nutrient-depleted (younger) bottom waters, as identified previously. Below that, d13C decreased continually to -0.5 per mil, about equal to the lowest LGM d13C in the North Pacific Ocean. Seven pairs of benthic and planktonic foraminiferal 14C dates from cores >2.5 km deep differ by 1100 ± 340 years, with a maximum apparent ventilation age of ~1500 years at 4250 m and at ~4700 m. Apparent ventilation ages are presently unavailable for the LGM < 2.5 km because of problems with reworking on the continental slope when sea level was low. Because LGM d13C is about the same in the deep North Atlantic and the deep North Pacific, and because the oldest apparent ventilation ages in the LGM North Atlantic are the same as the North Pacific today, it is possible that the same water mass, probably of southern origin, flowed deep within each basin during the LGM. Very early in the YD, dated here at 11.25 ± 0.25 (n = 10) conventional 14C kyr BP (equal to 12.9 calendar kyr BP), apparent ventilation ages <2.3 km water depth were about the same as North Atlantic Deep Water today. Below ~2.3 km, four YD pairs average 1030 ± 400 years. The oldest apparent ventilation age for the YD is 1600 years at 4250 m. This strong contrast in ventilation, which indicates a front between water masses of very different origin, is similar to glacial profiles of nutrient-like proxies. This suggests that the LGM and YD modes of ocean circulation were the same.

Silicon isotopes of sediment core GeoB2107-3

The supply of nutrients to the low-latitude thermocline is largely controlled by intermediate-depth waters formed at the surface in the high southern latitudes. Silicic acid is an essential macronutrient for diatoms, which are responsible for a significant portion of marine carbon export production. Changes in ocean circulation, such as those observed during the last deglaciation, would influence the nutrient composition of the thermocline and, therefore, the relative abundance of diatoms in the low latitudes. Here we present the first record of the silicic acid content of the Atlantic over the last glacial cycle. Our results show that at intermediate depths of the South Atlantic, the silicic acid concentration was the same at the Last Glacial Maximum (LGM) as it is today, overprinted by high silicic acid pulses that coincided with abrupt changes in ocean and atmospheric circulation during Heinrich Stadials and the Younger Dryas. We suggest these pulses were caused by changes in intermediate water formation resulting from shifts in the subpolar hydrological cycle, with fundamental implications for the nutrient supply to the Atlantic.

Age determination and Sea surface temperature reconstruction for sediment core H214

We present sea surface temperature (SST) records with centennial-scale resolution from the Bay of Plenty, north of New Zealand. Foraminiferal assemblage-based paleo-SST estimates provide a deglacial record of SST since 16.5 14C ka. Average Holocene SSTs are 15.6°C for winter and 20.3°C for summer, whereas average glacial values were 14.2°C for winter and 19.5°C for summer. Compared to modern time, cooling of SSTs at the Last Glacial Maximum (LGM) was ~0.9°C in winter and ~1.5°C in summer. The shift from glacial to Holocene temperatures began at 14.25 14C ka, warming by ~2°C until 12.85 14C ka when temperatures dipped back to glacial values at 11.65 14C ka. The timing of this return to glacial-like SST correlates well with the Antarctic Cold Reversal (ACR) rather than the Younger Dryas and documents that the influence of the ACR extended into the subtropics of the Southern Hemisphere, at least in this region of the southwest Pacific. By 10.55 14C ka an SST maximum in summer SSTs of up to 3°C warmer than modern occurred (?24°C), after which SST dropped, remaining at present-day temperatures since 9.3 14C ka. This early Holocene climatic optimum has been widely noted in the Southern Ocean, and this record indicates that this phenomenon also extended into the subtropics to the north of New Zealand.