Antarctic Intermediate Water penetration into the Northern Indian Ocean during the last deglaciation (original) (raw)
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Paleoceanography, 2004
1] Deep-sea sediment core FR1/97 GC-12 is located 990 mbsl in the northern Tasman Sea, southwest Pacific, where Antarctic Intermediate Water (AAIW) presently impinges the continental slope of the southern Great Barrier Reef. Analysis of carbon (d 13 C) and oxygen (d 18 O) isotope ratios on a suite of planktonic and benthic foraminifera reveals rapid changes in surface and intermediate water circulation over the last 30 kyr. During the Last Glacial Maximum, there was a large d 13 C offset (1.1%) between the surface-dwelling planktonic foraminifera and benthic species living within the AAIW. In contrast, during the last deglaciation (Termination 1), the d 13 C planktonic-benthic offset reduced to 0.4% prior to an intermediate offset (0.7%) during the Holocene. We suggest that variations in the dominance and direction of AAIW circulation in the Tasman Sea, and increased oceanic ventilation, can account for the rapid change in the water column d 13 C planktonic-benthic offset during the glacial-interglacial transition. Our results support the hypothesis that intermediate water plays an important role in propagating climatic changes from the polar regions to the tropics. In this case, climatic variations in the Southern Hemisphere may have led to the rapid ventilation of deep water and AAIW during Termination 1, which contributed to the postglacial rise in atmospheric CO 2 .
Nature Communications, 2013
The Southern Ocean plays a prominent role in the Earth's climate and carbon cycle. Changes in the Southern Ocean circulation may have regulated the release of CO 2 to the atmosphere from a deep-ocean reservoir during the last deglaciation. However, the path and exact timing of this deglacial CO 2 release are still under debate. Here we present measurements of deglacial surface reservoir 14 C age changes in the eastern Pacific sector of the Southern Ocean, obtained by 14 C dating of tephra deposited over the marine and terrestrial regions. These results, along with records of foraminifera benthic-planktic 14 C age and d 13 C difference, provide evidence for three periods of enhanced upwelling in the Southern Ocean during the last deglaciation, supporting the hypothesis that Southern Ocean upwelling contributed to the deglacial rise in atmospheric CO 2. These independently dated marine records suggest synchronous changes in the Southern Ocean circulation and Antarctic climate during the last deglaciation.
Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores.
Paleoceanography, 2002
The last glacial to interglacial transition was studied using down core records of stable isotopes in diatoms and foraminifera as well as surface water temperature, sea ice extent, and ice-rafted debris (IRD) concentrations from a piston core retrieved from the Atlantic sector of the Southern Ocean. Sea ice is the first variable to change during the last deglaciation, followed by nutrient proxies and sea surface temperature. This sequence of events is independent of the age model adopted for the core. The comparison of the marine records to Antarctic ice CO2 variation depends on the age model as 14C determinations cannot be obtained for the time interval of 29.5–14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B.P. lead the increase in atmospheric pCO2 by approximately 2000 years. Our diatom-based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO2. If gas exchange played a major role in determining glacial to interglacial CO2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO2 change must be sought elsewhere, rather than in the Southern Ocean.
The Paleocene-Eocene transition in the Antarctic Indian Ocean: Inference from planktic foraminifera
Lu, G. and Keller, G., 1993. The Paleocene-Eocene transition in the Antarctic Indian Ocean: Inference from planktic foraminifera. Mar. Micropaleontol., Isotopic depth stratification and relative abundance studies of planktic foraminifera at ODP Site 738 reveal three major faunal turnovers during the latest Paleocene and early Eocene, reflecting the climatic and structural changes in the Antarctic surface ocean.
Earth and Planetary Science Letters, 2012
The use of neodymium (Nd) isotopes to reconstruct past water mass mixing relies upon the quasiconservative behaviour of this tracer, whereas recent studies in the modern oceans have suggested that boundary exchange, involving the addition of Nd from ocean margin sediments, may be an important process in the Nd cycle. Here we suggest that the relative importance of water mass advection versus boundary exchange can be assessed where the deep western boundary current in the Indian Ocean flows past the Madagascan continental margin; a potential source of highly unradiogenic Nd. Foraminiferal coatings and bulk sediment reductive leachates are used to reconstruct bottom water Nd isotopic composition (e Nd ) in 8 Holocene age coretops, with excellent agreement between the two methods. These data record spatial variability of $ 4 e Nd units along the flow path of Circumpolar Deep Water; e Nd E À 8.8 in the deep southern inflow upstream of Madagascar, which evolves towards e Nd E À 11.5 offshore northern Madagascar, whereas e Nd E À 7.3 where deep water re-circulates in the eastern Mascarene Basin. This variability is attributed to boundary exchange and, together with measurements of detrital sediment e Nd , an isotope mass balance suggests a deep water residence time for Nd of r 400 yr along the Madagascan margin. Considering deglacial changes, a core in the deep inflow upstream of Madagascar records e Nd changes that agree with previous reconstructions of the Circumpolar Deep Water composition in the Southern Ocean, consistent with a control by water mass advection and perhaps indicating a longer residence time for Nd in the open ocean away from local sediment inputs. In contrast, sites along the Madagascan margin record offset e Nd values and reduced glacial-interglacial variability, underlining the importance of detecting boundary exchange before inferring water mass source changes from Nd isotope records. The extent of Madagascan boundary exchange appears to be unchanged between the Holocene and Late Glacial periods, while a consistent shift towards more radiogenic e Nd values at all sites in the Late Glacial compared to the Holocene may represent a muted signal of a change in water mass source or composition.
Geophysical Research Letters
The relationship between ocean circulation and monsoon systems over orbital to sub-millennial timescales is a crucial but poorly-constrained component of the climate system. Here, using foraminiferal and detrital neodymium (Nd) isotope records from the intermediate-depth northern Indian Ocean, we provide new evidence revealing that both monsoon-driven weathering inputs and water mass advection from the Southern Ocean influenced past seawater Nd isotope changes in this region. Our results suggest that Indian Summer Monsoon weakening coincided with enhanced northward Antarctic Intermediate Water (AAIW) advection during the last deglaciation, reflecting a strong interhemispheric coupling. In contrast, the Early Holocene was characterised by enhanced monsoon strength but persistently strong AAIW inflow, indicating a relationship in the opposite sense. These differing interhemispheric relationships indicate asynchronous changes in the global atmosphereoceanclimate system, and may represent a previously unrecognised component of the ocean-atmosphere reorganization during the deglacial to Holocene transition.
Climate of the Past
We present downcore records of redox-sensitive authigenic uranium (U) and manganese (Mn) concentrations based on five marine sediment cores spanning a meridional transect encompassing the Subantarctic and Antarctic zones in the southwestern Indian Ocean covering the last glacial cycle. These records signal lower bottom water oxygenation during glacial climate intervals and generally higher oxygenation during warm periods, consistent with climaterelated changes in deep-ocean remineralized carbon storage. Regional changes in the export of siliceous phytoplankton to the deep sea may have entailed a secondary influence on oxygen levels at the water-sediment interface, especially in the Subantarctic Zone. The rapid reoxygenation during the deglaciation is in line with increased ventilation and enhanced upwelling after the Last Glacial Maximum (LGM), which in combination conspired to transfer previously sequestered remineralized carbon to the surface ocean and the atmosphere, contributing to propel the Earth's climate out of the last ice age. These records highlight the still insufficiently documented role that the Southern Indian Ocean played in the air-sea partitioning of CO 2 on glacialinterglacial timescales.
No Change in Southern Ocean Circulation in the Indian Ocean From the Eocene Through Late Oligocene
Paleoceanography and Paleoclimatology, 2018
Deciphering the evolution of Southern Ocean circulation during the Eocene and Oligocene has important implications for understanding the development of the Antarctic Circumpolar Current and transition to Earth's "icehouse" climate. To better understand ocean circulation patterns in the Indian Ocean sector of the Southern Ocean, we generated a new fossil fish tooth neodymium isotope record (ε Nd) from the upper Eocene to upper Oligocene sections (36-23 Ma) of Ocean Drilling Program Sites 744 and 748 (Kerguelen Plateau, Indian Ocean). Reconstructed seawater ε Nd values from fossil fish teeth are used to trace changes in water masses across ocean basins. The records from Site 748 and Site 744 reveal a gradual shift from ε Nd values around À6.5 to À7.5 in the late Eocene to ε Nd values between À7.5 and À8.3 by the late Oligocene, consistent with a Circumpolar Deep Water (CDW) influence at the Kerguelen Plateau throughout the Oligocene. We interpret the shift to less radiogenic values to reflect the increased export of Northern Component Water to the Southern Ocean, likely into the proto-CDW. However, the records show no major change in water mass composition around the Kerguelen Plateau that would accompany an increase in Pacific throughflow related to the opening of Drake Passage and imply that Pacific throughflow via the Drake Passage occurred by the late Eocene. High-frequency variability in ɛ Nd values at Site 744 is interpreted as an imprint of Oligocene glacial activity, with a particularly pronounced excursion at 32.6 Ma roughly coinciding with other glacial weathering indicators around Antarctica.