Strong glacial-interglacial variability in upper ocean hydrodynamics, biogeochemistry, and productivity in the southern Indian Ocean (original) (raw)
Related papers
Indian Ocean circulation and productivity during the last glacial cycle
Earth and Planetary Science Letters, 2009
The Indian Ocean is an important part of the global thermohaline circulation system, receiving deep waters sourced from the Southern Ocean and being the location of upwelling and surface-ocean current flow, which returns warm and salty waters to the Atlantic. It is also an ideal location to reconstruct the link between thermohaline circulation and deep-water nutrient contents. No mixing occurs between major deep-water masses along flow paths within the Indian Ocean, so changes in water-mass provenance reflect changes in deep-ocean circulation while nutrient contents reflect addition and dissolution of organic matter. We present neodymium (Nd) and carbon (C) isotope records, proxies of water-mass provenance and nutrient contents, respectively, from an equatorial Indian Ocean core (SK129−CR2) spanning the last 150 kyr. The Nd isotope record shows that an increased proportion of North Atlantic Deep Water (NADW) reached the Indian Ocean during interglacials (marine isotope stages, MIS 1 and 5), and a reduced proportion during glacials (MIS 2, 4, and 6), and also that changes occurred during MIS 3. The magnitude and timing of deglacial and some MIS 3 variability is very similar to those in the RC11−83/TNO57−21 South Atlantic deep Cape Basin Nd isotope record, suggesting that Atlantic meridional overturning circulation changes were effectively propagated from the southeastern Atlantic into the central Indian Ocean via the Southern Ocean. Comparison of the Nd and C isotope records shows that deep-ocean circulation was decoupled from nutrient-content changes on glacial−interglacial timescales, in particular suggesting higher productivity during MIS 5.
Climate of the Past Discussions
The position of subtropical front (STF), Agulhas Current (AC) and Agulhas Return Current (ARC) controls the hydrography of southwestern Indian Ocean. Although, equatorward migration of STF and reduction in Agulhas leakage has been reported during the last glacial period, the fate of ARC during the last glacial-interglacial cycle is not clear. Therefore, in order to understand changes in the position and strength of ARC during the last glacial-interglacial cycle, here we reconstruct hydrographic changes in the southwestern Indian Ocean from temporal variation in planktic foraminiferal abundance, stable isotopic ratio (δ 18 O) and trace metal ratio (Mg/Ca) of planktic foraminifera Globigerina bulloides in a core collected from the Agulhas Retroflection Region (ARR) in the southwestern Indian Ocean. Increased abundance of G. bulloides suggests that the productivity in the southwestern Indian Ocean increased during glacial period which confirms previous reports of high glacial productivity in the Southern Ocean. The increased productivity was likely driven by a combination of equator-ward migration of subtropical front and westerlies. Increase in relative abundance of Neogloboquadrina pachyderma Dextral suggests warming of ARR leading to strong thermocline in the southwestern Indian Ocean during the last glacial period. We suggest that the warming of Agulhas Retroflection Region was driven by strengthened ARC which shifted to the east of its present location, thus bringing warmer and saltier water to the southwestern Indian Ocean. Therefore, it is inferred that over the last glacial-interglacial cycle, the hydrography of southwestern Indian Ocean was driven by an eastward shift of retroflection region as well as migrating subtropical front.
Climate of The Past, 2022
We have measured Cd/Ca ratios of several benthic foraminiferal species and studied benthic foraminiferal assemblages on two cores from the northern Indian Ocean (Arabian Sea and northern Bay of Bengal, BoB), in order to reconstruct variations in intermediate-water circulation and paleo-nutrient content since the last deglaciation. Intermediate water Cd w records estimated from the benthic Cd/Ca reflect past changes in surface productivity and/or intermediate-bottom-water ventilation. The benthic foraminiferal assemblages are consistent with the geochemical data. These results suggest that during the last deglaciation, Cd w variability was primarily driven by changes in intermediate-water properties, indicating an enhanced ventilation of intermediate-bottom water masses during both Heinrich Stadial 1 and the Younger Dryas (HS1 and YD, respectively). During the Holocene, however, surface primary productivity appears to have influenced Cd w more than intermediate water mass properties. This is evident during the early Holocene (from 10 to 6 cal ka) when benthic foraminiferal assemblages indicate that surface primary productivity was low, resulting in low intermediate-water Cd w at both sites. Then, from ∼ 5.2 to 2.4 cal ka, surface productivity increased markedly, causing a significant increase in the intermediate-water Cd w in the southeastern Arabian Sea and the northeastern BoB. The comparison of intermediatewater Cd w records with previous reconstructions of past Indian monsoon evolution during the Holocene suggests a direct control of intermediate-water Cd w by monsoon-induced changes in upper-water stratification and surface primary productivity.
Late Pleistocene surface and thermocline conditions of the eastern tropical Indian Ocean
Quaternary Science Reviews, 2010
Surface and thermocline conditions of the eastern tropical Indian Ocean were reconstructed through the past glacial-interglacial cycle by using Mg/Ca and alkenone-paleothermometry, stable oxygen isotopes of calcite and seawater, and terrigenous fraction performed on sediment core GeoB 10038-4 off SW Sumatra (w6 S, 103 E, 1819 m water depth). Results show that annual mean surface and thermocline temperatures varied differently and independently, and suggest that surface temperatures have been responding to southern high-latitude climate, whereas the more variable thermocline temperatures were remotely controlled by changes in the thermocline temperatures of the North Indian Ocean. Except for glacial terminations, salinity proxies indicate that changing intensities of the boreal summer monsoon did not considerably affect annual mean conditions off Sumatra during the past 133,000 years. Our results do not show a glacial-interglacial pattern in the thermocline conditions and reject a linear response of the tropical Indian Ocean thermocline to mid-and high-latitude climate change. Alkenone-based surface temperature estimates varied in line with the terrigenous fraction of the sediment and the East Asian winter monsoon proxy records at the precession band suggestive of monsoon (sea level) to be the dominant control on alkenone temperatures in the eastern tropical Indian Ocean on sub-orbital (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.
Journal of Asian Earth Sciences, 2006
This study attempts to analyse paleoceanographic changes in the Central Indian Ocean (Deep Sea Drilling Project Site 237), linked to monsoon variability as well as deep-sea circulation during the Plio-Pleistocene. We used factor and cluster analyses of census data of the 34 most dominant species of benthic foraminifera that enabled us to identify five biofacies: Astrononion umbilicatulum-Uvigerina proboscidea (Au-Up), Pullenia bulloides-Bulimina striata (Pb-Bs), Globocassidulina tumida-Nuttallides umbonifera (Gt-Nu), Gyroidinoides nitidula-Cibicides wuellerstorfi (Gn-Cw) and Cassidulina carinata-Cassidulina laevigata (Cc-Cl) biofacies. Knowledge of the environmental preferences of modern deep-sea benthic foraminifera helped to interpret the results of factor and cluster analyses in combination with oxygen and carbon isotope values. The biofacies indicative of high surface productivity, resulting from a stronger South Equatorial Current (Au-Up and Pb-Bs biofacies), dominate the early Pliocene interval (5.6-4.5 Ma) of global warmth. An intense Indo-Pacific 'biogenic bloom' and strong Oxygen Minimum Zone extended to intermediate depths (w1000-2000 m) over large parts of the Indian Ocean in the early Pliocene. Since 4.5 Ma, the food supply in the Central Indian Ocean dropped and fluctuated while deep waters were corrosive (biofacies Gt-Nu, Gn-Cw). The Pleistocene interval is characterized by an intermediate flux of organic matter (Cc-Cl biofacies). q
Multiple proxy studies on sediment core SK 200/22a from the sub-Antarctic sector of the Indian Ocean revealed millennial-scale fluctuations in terrigenous input during the last 63,000 years. The marine isotope stages 1 (MIS 1) and MIS 3 are characterized by generally low concentrations of magnetic minerals, being dominated by fine-grained magnetite and titano-magnetite. Within the chronological constraints, periods of enhanced terrigenous input and calcite productivity over the last 63,000 years are nearly synchronous with the warming events recorded in Antarctic ice cores. An equatorward shift of the westerly wind system in association with a strengthening of the Antarctic Circumpolar Current (ACC) system may have promoted wind-induced shallow-water erosion around oceanic islands, leading to enhanced terrigenous input to the core site. Major ice-rafted debris events at 13–23, 25–30, 45–48 and 55–58 ka BP are asynchronous with δ18O and carbonate productivity records. This out-of-phase relation suggests that ice-sheet dynamics and ACC intensity were the primary factors influencing ice rafting and terrigenous input to the Indian sector of the Southern Ocean, with only limited support from sea-surface warming.
Reconstruction of palaeoproductivity from Southern Ocean is crucial for understanding the functioning of the Southern Ocean biological pump in the past. High resolution records of multi-proxy parameters (calcium carbonate, opal, total organic carbon biogenic barium and planktonic carbon isotope ratios (δ13C)) were investigated in two well-dated sediment cores (SK200/22a and SK200/27) from the Indian sector of Southern Ocean situated to the north and south of Antarctic Polar front (APF), respectively. The palaeoproductivity records extending 95 ka BP (SK200/22a) and 75 ka BP (SK200/27) revealed inverse relationships between the calcite and opal productivity, indicating the influence of shifting nutrient regimes. At core SK200/22a, reduced calcite productivity during marine isotope stage (MIS) 2, 4, and part of MIS 3 suggest an equatorward migration of the frontal regimes during glacial intervals. Compared to this, the region south of the APF (core SK200/27) was characterized by the near absence of calcite content during the last glacial period and increased opal productivity during MIS 1 and MIS 3, supporting a southward migration of APF during warmer intervals. Ba(bio) records exhibit good correlation with opal records in both the cores and also correlate with that of calcite record at SK200/ 22a, indicating that Ba is influenced by the combined opal and calcite productivity. The enhanced opal productivity during the glacial periods north of the APF is attributed to the northward shifting of oceanic fronts and associated transfer of nutrients. Diatom productivity records of SK200/22a reveal significant similarities with the dust records from the Antarctic and Southern Ocean, but showed no significant relationships with the diatom record of SK200/27. It is proposed that the dust-derived Fe input had apparently influenced the palaeoproductivity north of the modern APF, but had a minor influence on opal productivity south of the APF. Comparison with the ice core climate records from Antarctica and Greenland revealed that bioproductivity peaks in the study region are nearly synchronous with the millennial Antarctic warming events. Remarkably, the calcite and opal productivity records at SK200/22a responded differently to the Antarctic warming events, with opal productivity lagging behind the calcite productivity peaks by 1–2 ka.