Late Quaternary sea ice history in the Indian sector of the Southern Ocean as recorded by diatom assemblages (original) (raw)
Related papers
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 goal of this study is to assess the changes that have occurred during the Mid-Pleistocene Transition, an important transitional period in cryosphere evolution, by examining the siliceous microfossil record of sediments collected proximal to a major ice drainage outlet for the West Antarctic Ice Sheet. Core PS58/254 was collected from a sediment drift on the upper continental rise in the Amundsen Sea, directly offshore from Pine Island Bay, one of the three main discharge areas for the West Antarctic Ice Sheet (WAIS). Published data on physical properties, geochemical composition, grain size and clay mineral assemblages are complemented here by a high-resolution record (sample spacing 10 cm) of the siliceous microfossil assemblages (diatoms and silicoflagellates). Between 1200 ka and 621 ka, the assemblage is relatively diverse, with Actinocyclus ingens, Thalassiothrix antarctica and Fragilariopsis kerguelensis dominating the assemblages, but diatom abundance is variable from low to barren. Additionally, the occurrence of A. ingens, Thalassiosira elliptipora and Thalassiosira fasciculata is used to confirm and further refine the existing age model and extend it back to 1200 ka. Species composition during the last ca. 621 ka is dominated by F. kerguelensis, which consistently comprises 80-90% of the assemblage. A clear relationship between diatom abundance and glacial/interglacial variability is apparent after 621 ka, which resembles the glacial-interglacial variability previously observed in other proxy data. A significant change in both sediment composition and diatom assemblages is observed at 621 ka. This change concurs with the last abundant occurrence of A. ingens and the end of the Mid-Pleistocene Transition (MPT, i.e. the onset of modern eccentricity/precession-paced glacial cycles around 650 ka). We suggest that during interglacial periods after 621 ka the Amundsen Sea Low pressure system shifted seasonally southwestwards towards the shelf and thereby increased the advection of relatively warm Circumpolar Deep Water (CDW) onto the Amundsen Sea shelf, which is a major factor for present ice-sheet melting in this part of West Antarctica.
Antarctic sea ice over the past 130,000 years, Part 1: A review of what proxy records tell us
Antarctic sea ice plays a critical role in the Earth system, influencing energy, heat and freshwater fluxes, airsea gas exchange, ice shelf dynamics, ocean circulation, nutrient cycling, marine productivity and global carbon cycling. However, accurate simulation of recent sea-ice changes remains challenging and, therefore, projecting future sea-ice changes and their influence on the global climate system is uncertain. Reconstructing past changes in sea-ice cover can provide additional insights into climate feedbacks within the Earth system at different timescales. This paper is the first of two review papers from the Cycles of Sea Ice Dynamics in the Earth system (C-SIDE) working group. In this first paper, we review marine-and ice core-based sea-ice proxies and reconstructions of sea-ice changes throughout the last glacialinterglacial cycle. Antarctic sea-ice reconstructions rely mainly on diatom fossil assemblages and highly branched isoprenoid (HBI) alkenes in marine sediments, supported by chemical proxies in Antarctic ice cores. Most reconstructions for the Last Glacial Maximum (LGM) suggest that winter sea ice expanded all around Antarctica and covered almost twice its modern surface extent. In contrast, LGM summer sea ice expanded mainly in the regions off the Weddell and Ross seas. The difference between winter and summer sea ice during the LGM led to a larger seasonal cycle than today. More recent efforts have focused on reconstructing Antarctic sea ice during warm periods, such as the Holocene and the Last Interglacial (LIG), which may serve as an analogue for the future. Notwithstanding regional heterogeneities, existing reconstructions suggest that sea-ice cover increased from the warm mid-Holocene to the colder Late Holocene with pervasive decadal-to millennial-scale variability throughout the Holocene. Studies, supported by proxy modelling experiments, suggest that sea-ice cover was halved during the warmer LIG when global average temperatures were ∼ 2 • C above the pre-industrial (PI). There are limited marine (14) and ice core (4) sea-ice proxy records covering the complete 130 000 year (130 ka) last glacial cycle. The glacial-interglacial pattern of sea-ice advance and retreat appears relatively similar in each basin of the Southern Ocean. Rapid retreat of sea ice occurred during Terminations II and I while the expansion of sea ice during the last glaciation appears more gradual especially in ice core data sets. Marine records suggest that the first prominent expansion occurred during Marine Isotope Stage (MIS) 4 and that sea ice reached maximum extent during MIS 2. We, however, note that additional sea-ice records and transient model simulations are required to better identify the underlying drivers and feedbacks of Antarctic sea-ice changes over the last 130 ka. This understanding is critical to improve future predictions.
A diatom record of late Pliocene cooling from the Ross Sea continental shelf, AND-1B, Antarctica
Global and Planetary Change, 2012
A late Pliocene-early Pleistocene, 2.9-2.0Ma, diatom record from the Antarctic Geological Drilling Program (ANDRILL) MIS drillcore AND-1B is presented. This core, recovered from beneath the Ross Ice Shelf south of Ross Island, comprises multiple diatomaceous-sediment units deposited during interglacial periods with open water over the core site. These represent interglacial phases of orbitally paced climate cycles and are punctuated by glacial advances. Extant diatom assemblages have limited presence in the late Pliocene record, which makes environmental interpretation less straight forward. We employ modern ecological data in combination with late Pliocene to present variation in diatom assemblages across the Southern Ocean oceanic fronts based on DSDP/ ODP diatom biostratigraphic data to evaluate paleoenvironmental change for the 2.9-2.0Ma interval of the AND-1B core. The diatom assemblages from AND-1B record a progressive environmental change through the late Pliocene-early Pleistocene. A relatively warm period with potential SST of up to 4°C at ca. 2.9Ma was succeeded by a reduction of warm water species and an increase of taxa associated with more southerly water masses until 2.58Ma (at isotope stage G1). Younger, early Pleistocene, diatomaceous units are dominated by extinct Rouxia, Thalassiosira species and newly described Fragilariopsis species indicative of cold open water and drift ice. The last recorded cooling step occurs at the top of the interval studied (ca 2.0) Ma indicated by the trace abundance of Fragilariopsis and Actinocyclus species present in modern sea ice assemblages but absent in the late Plioceneearly Pleistocene AND-1B record, even though many of its species had their first occurrences during the Pliocene. The extant sea ice assemblage occurs with an abundance of 2-29% (average 10%) in the late Pliocene-early Pleistocene AND-1B record implying that the modern sea ice and ice shelf polar conditions were not established within the studied interval. The most frequently applied sea ice indicator, Fragilariopsis curta, is consistently present in low to moderate abundance (1-22%) together with Chaetoceros resting spores (2-30%) through the 2.9-2.0Ma interval. The diatom assemblage shifts indicate a dynamic environment with an overall trend towards colder conditions after ca. 2.6Ma but does not reach the Holocene configuration.
Global and Planetary Change, 1997
A new theory is proposed to explain global cooling at the onset of Pleistocene glacial periods. Atmospheric CO 2 drawdown is considered to be the driving force behind global cooling, brought about by heightened productivity at the equatorial divergences and along continental margins, particularly in upwelling regions. Eutrophication appears to be triggered when global warming during late interglacial periods causes accelerated melting of the West Antarctic Ice Sheet. This would release large reserves of silicate-enriched subglacial meltwaters into the surrounding oceans where entrainment would take place into deep and intermediate currents forming in Antarctic and subantarctic waters. Subsequent advection, mixing and upweiling of silicate-enriched deep and intermediate waters into the coastal zones and open-ocean divergences results in the proliferat:ion of large, rapidly-sinking diatom species with a high affinity for dissolved silicate. These blooms enhance rates of recycling of N and P in upwelling regions and accelerate rates of organic carbon production, export and sequestration in shelf and slope sediments and in the deep sea. The resultant atm. CO 2 drawdown initiates global cooling. Consequent expansion of Northern Hemisphere glaciers lowers sea level, while increased temperature and pressure gradients between equatorial and polar regions intensify meridional winds. The former process exposes nutrient-enriched coastal sediments to wave erosion, thereby releasing new nutrient supplies, while the latter process enhances upwelling. The combined effect is to greatly increase rates of org. C production and export from continental margins and further accelerate atm. CO 2 drawdown. Glacial-period cooling is also enhanced by a number of other positive feedbacks, including changes in albedo, water vapour and cloud cover. Episodic warming intervals during glacial periods may be related to insolation changes associated wil:h orbital precession and tilt cycles, but processes involved in deglaciation and reversion to the interglacial climatic regime are complex and not yet fully understood.
Antarctic Intermediate Water penetration into the Northern Indian Ocean during the last deglaciation
Earth and Planetary Science Letters, 2018
The two-stage increase in atmospheric carbon dioxide (CO 2), and the associated decrease in radiocarbon (14 C) during the last deglaciation, are thought to have been linked to enhanced Southern Ocean upwelling and the rapid release of sequestered 14 C-depleted CO 2. Antarctic Intermediate Water (AAIW), originating from the Southern Ocean, reflects variations in the Southern Ocean and, crucially, mirrors the chemical signature of upwelling deep water. However, the penetration of AAIW into the Northern Indian Ocean and its relationship with deglacial climate changes have not been thoroughly elucidated to date. Here, we present the neodymium isotopic composition (ε Nd) of mixed planktonic foraminifera from core MD77-176 from an intermediate depth in the Northern Indian Ocean to reconstruct the past evolution of intermediate water during deglaciation. The ε Nd record in the Northern Indian Ocean displays two pulse-like shifts towards more radiogenic Southern Ocean values during the deglaciation, and these shifts coincide with excursions in 14 C and ε Nd records in the Pacific and Atlantic Oceans. These results suggest invasion of AAIW into the Northern Hemisphere oceans associated with enhanced Southern Ocean ventilation during deglaciation. Our new ε Nd record strongly supports the close linkage of AAIW propagation and atmospheric CO 2 rise through Southern Ocean ventilation during deglaciation.
Global Biogeochemical Cycles, 2005
1] In an effort to investigate the controlling factors behind Holocene d 13 C org changes in East Antarctica we report high-resolution down-core records of bulk organic matter carbon isotopic ratios (d 13 C org ), diatom census counts, total organic carbon, and biogenic silica content taken from one core recovered in the Adélie Trough. A good correspondence between the d 13 C org record and records of small/large and pennate/centric ratios in many of the core segments indicates that diatom species composition affected Holocene d 13 C org variations, possibly via diatom shape and size effects. Variations in the surface water CO 2 concentration and in the isotopic composition of the source during the Holocene cannot be ruled out although they cannot explain rapid and large-amplitude d 13 C org changes. Within the limit of our investigation, our results argue against active carbon acquisition through a carbon concentration mechanism as evidenced in low-latitude upwelling systems. The bulk organic matter thus represents a mixing of diatom taxa having different fractionation affinity to aqueous CO 2 . Our results confirm previous evidence which demonstrates that d 13 C org down-core records should be used with great caution to reconstruct past CO 2 content in surface waters.
Paleoceanography, 1992
A set of numerical equations is developed to estimate past sea surface temperatures (SST) from fossil Antarctic diatoms. These equations take into account both the biogeographic distribution and experimentally derived silica dissolution. The data represent a revision and expansion of a floral data base used previously and includes samples resulting from progressive opal dissolution experiments. Factor analysis of 166 samples (124 Holocene core top and 42 artificial samples) resolved four factors. Three of these factors depend on the water mass distribution (one Subantarctic and two Antarctic assemblages); factor 4 corresponds to a "dissolution assemblage". Inclusion of this factor in the data analysis minimizes the effect of opal dissolution on the assemblages and 1D6partement de G6ologie et Oc•anographie, CNRS URA. 197, Universit6 de Bordeaux 1, Paper number 92PA00709. 0883-8305/92/92PA-00709510.00 gives accurate estimates of SST over a wide range of biosiliceous dissolution. A transfer function (DTF 166/34/4) is derived from the distribution of these factors versus summer SST. Its standard error is + IøC in the-1 to +10 øC summer temperature range. This transfer function is used to estimate SST changes in two southern ocean cores (43øS and 55øS) which cover the last climatic cycle. The time scale is derived from the changes in foraminiferal oxygen and carbon isotopic ratios. The reconstructed SST records present strong analogies with the air temperature record over Antarctica at the Vostok site, derived from changes in the isotopic ratio of the ice. This similarity may be used to compare the oceanic isotope stratigraphy and the Vostok time scale derived from ice flow model. The oceanic time scale, if taken at face value, would indicate that large changes in ice accumulation rates occurred between warm and cold periods. 90006,Paleoclimatological and chronological implications of the Vostok core dust record, Nature, 343, 56-58, 1990. Pichon, J.J., M. Labracherie, L.D. Labeyrie, and J. Duprat, Transfer functions between diatom assemblages and surface hydrology in the 318 southern ocean, Paleogeogr. Paleoclimatol. Paleoecol., 61, 79-95, 1987. A seasonally recurrent patch of Antarctic planktonic diatoms, Search, 16(1-2), 48, 1985. Robinson, S.G., The late Pleistocene paleoclimatic record of north atlantic deep sea sediments revealed by mineral magnetic measurements, Phys. Earth Planet Inter., 42, 22-47, 1986. Shackleton, N.J., Attainment of isotopic equilibrium between ocean water and the benthonic foraminifera genus Uvigerina: Isotopic changes in the ocean during the last glacial, in les mdthodes quantitatives d'•tude des variations du climat au cours du pleistocene, edited by J. Labeyrie, pp. 203-210, Colloque CNRS
Evidence from diatoms for Holocene climate fluctuation along the East Antarctic margin
Holocene, 2001
Diatom assemblages in two Holocene sediment cores (GC1 and GC2) from the Mac. Robertson Shelf, East Antarctica, are compared with modern sedimentary diatom assemblages from the same area. Open marine deposition commenced in Iceberg Alley (GC1), on the outer continental shelf, Ͼ10.7 adj. 14 C kyr BP. Chaetoceros resting spores, which may indicate water-column stabilization from melting glacial and/or sea ice or the maximum summer sea-ice retreat, dominate the diatom assemblage. Approximately 7.5 adj. 14 C kyr BP, a sea-ice diatom assemblage was deposited. This assemblage is similar to that being deposited in the surface sediments of the Mac. Robertson Shelf today and suggests that perennial sea ice has persisted in the vicinity of Iceberg Alley since that time. Interbedded within the sea-ice assemblage, however, are Corethron-rich sediment layers that suggest mid-to late-Holocene high-productivity events associated with a climatic optimum. The diatom record from Nielsen Basin (GC2), on the inner continental shelf, is relatively uniform compared to that in GC1. Glacial ice was present over the region c. Ͼ5.6 adj. 14 C kyr BP and a dissolution diatom assemblage was deposited beneath it. Following ice retreat, an ice-edge diatom assemblage was deposited briefly before sea-ice conditions similar to that on the continental shelf today developed. There is no evidence in GC2 for the mid-to late-Holocene high-productivity events identified in GC1.