Shifting frontal regimes and its influence on bioproductivity variations during the late Quaternary in the Indian sector of Southern Ocean (original) (raw)
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Two Modes of Change in Southern Ocean Productivity Over the Past Million Years
Science, 2013
Export of organic carbon from surface waters of the Antarctic zone of the Southern Ocean decreased during the last ice age, coinciding with declining atmospheric CO2 concentrations, signaling reduced exchange of CO2 between the ocean interior and the atmosphere. In contrast, in the Subantarctic Zone, export production increased into ice ages coinciding with rising dust fluxes and thus suggesting iron fertilization of Subantarctic phytoplankton. Here, a new high-resolution productivity record from the Antarctic zone is compiled with parallel Subantarctic data over the last million years. Together, they fit the view that the combination of these two modes of Southern Ocean change determines the temporal structure of the glacial/interglacial atmospheric CO2 record, including during the interval of "lukewarm" interglacials between 450 and 800 thousand years ago. Antarctic ice core measurements reveal that regional air temperatures and atmospheric pCO 2 were tightly correlated over glacial-interglacial cycles of the past 800 kyrs (1). Many studies have inferred a dominant role for the Southern Ocean in modulating glacial-interglacial variability of atmospheric pCO 2 ((2) and references therein). The central role of the Southern Ocean is thought to reflect its leverage on the global efficiency of the biological pump, in which the production, sinking, and deep
Middle Eocene to Late Oligocene Antarctic glaciation/deglaciation and Southern Ocean productivity
Paleoceanography, 2014
During the Eocene-Oligocene transition, Earth cooled significantly from a greenhouse to an icehouse climate. Nannofossil assemblages from Southern Ocean sites enable evaluation of paleoceanographic changes and, hence, of the oceanic response to Antarctic ice sheet evolution during the Eocene and Oligocene. A combination of environmental factors such as sea surface temperature and nutrient availability is recorded by the nannofossil assemblages of and can be interpreted as responses to the following changes. A cooling trend, started in the Middle Eocene, was interrupted by warming during the Middle Eocene Climatic optimum and by short cooling episodes. The cooling episode at 39.6 Ma preceded a shift toward an interval that was dominated by oligotrophic nannofossil assemblages from~39.1 to~36.2 Ma. We suggest that oligotrophic conditions were associated with increased water mass stratification, low nutrient contents, and high efficiency of the oceanic biological pump that, in turn, promoted sequestration of carbon from surface waters, which favored cooling. After 36.2 Ma, we document a large synchronous surface water productivity turnover with a dominant eutrophic nannofossil assemblage that was accompanied by a pronounced increase in magnetotactic bacterial abundance. This turnover reflects a response of coccolithophorids to changed nutrient inputs that was likely related to partial deglaciation of a transient Antarctic ice sheet and/or to iron delivery to the sea surface. Eutrophic conditions were maintained throughout the Oligocene, which was characterized by a nannofossil assemblage shift toward cool conditions at the Eocene-Oligocene transition. Finally, a warm nannofossil assemblage in the Late Oligocene indicates a warming phase.
Open-File Report
Core JPC17B was collected from the Adélie-Drift, a nearly 300-m thick drift deposit at 140ºE along the Indian Ocean sector of the Antarctic continental shelf. Sediments consist of nearly continuously laminated diatom mud and diatom ooze, with accumulation rates on the order of 20-21 m kyr-1 based on 10 AMS radiocarbon dates. Spectral analysis was performed on time series of opal content, d 13 C org , Ti content, and Ba content. Strong decadal to centuryscale variability is present in all proxies. Opal and Ti-time series show strong variance at periods of 11, 22 and 110years, which are suggestive of solar forcing. Ba-time series exhibits strong variance at a period of 3-3.6-yr, consistent with possible El Niño-Southern-Oscillation (ENSO) forcing.
Paleoceanography, 2000
Various biomarkers (n-alkanes, n-alcohols, and steroIs) have been studied in a piston core TSP-2PC taken from the Southern Ocean to reconstruct the paleoenvironmental changes in the subantarctic region for the last two deglaciations. Mass accumulation rates of terrestrial (higher molecular weight n-alkanes and n-alcohols) and marine (dinosterol and brassicasterol) biomarkers increased significantly at the last two glacials and stayed low during interglacial peaks (early Holocene and the Eemian). These records indicate that the enhanced atmospheric transport of continental materials and the increased marine biological productivity were synchronously linked in the Southern Ocean at the last two glacials. This suggests that increased glacial dust inputs have relieved iron limitation in the subantarctic Southern Ocean. These two processes, however, were not linked at the cooling phase from the Eemian to marine isotope stage (MIS) 5d. During this period, paleoproductivity may have been influenced by the latitudinal migration of the highproduction zone associated with the Antarctic Polar Front.
Late Quaternary changes in biogenic opal fluxes in the Southern Indian Ocean
Marine Geology, 2003
Late Quaternary sedimentary and paleoenvironmental conditions in the southern Indian Ocean have been reconstructed from radioisotope and proxy element profiles (biogenic opal and organic carbon) measured on five sediment cores taken along a transect across the Indian sector of the Antarctic Circumpolar Current. Dissolutioncorrected opal rain rates were used to reconstruct past changes of opal productivity for this region. Records from these five cores indicate that opal productivity during glacial periods was lower than presently recorded south of the Antarctic Polar Front (APF), probably due to increased ice cover. North of APF, opal productivity was slightly greater during glacial periods than during the Holocene, probably in response to (1) the northward migration of the APF by approximately 5 ‡ latitude, (2) a northward transport of Si from the Antarctic Zone, and (3) an increase of Fe, necessary for opal-producing organisms, via upwelling and the erosion of the Kerguelen Plateau. We also invoke a decoupling between opal burial and organic carbon flux to the seabed to explain the variation in buried Si/C ratio between glacial and interglacial sediment. This decoupling is principally explained by better organic carbon preservation in the glacial sediments due to strong sediment focussing. An increase in glacial export paleoproductivity is not supported by the data, implying that bioproductivity variations in the Southern Indian Ocean are unlikely to have contributed to the glacial drawdown of atmospheric CO 2 inferred from ice core data. ß
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.
Paleoceanography, 2004
1] Isotopic measurements of diatom-bound nitrogen, using a wet chemical oxidation combined with the ''denitrifier'' method for nitrate analysis, show significant offsets from previously published combustion-based measurements. This offset is attributed to a gaseous nitrogen blank associated with the diatom's opal frustule. Moreover, experimentation with multiple chemical cleaning protocols demonstrates that diatom microfossils from the clay-rich sediments of the glacial Antarctic are more difficult to clean than Holocene materials. New downcore profiles from the Antarctic show no change in the diatom-bound N 15 N/ 14 N between the last glacial and the Holocene in the Atlantic sector, and the elevation of glacial diatom-bound 15 N/ 14 N relative to the Holocene in the Indian sector is smaller than in previous measurements. These data suggest no change in the degree of nitrate utilization in the Atlantic sector and at most a 20% increase (from $25 to 45%) in the Indian sector. The new measurements suggest that, during the last ice age in the Atlantic sector of the Antarctic, the atmospheric source of biologically available iron was not so great as to become significant relative to the iron supply from below. Given the apparent spatial variability in the degree of nitrate drawdown, more work is required to develop an adequate picture of the glacial Antarctic nutrient field.
Global and Planetary Change, 2004
During Leg 177 of the Ocean Drilling Program (ODP), a well-preserved middle Eocene to lower Miocene sediment record was recovered at Site 1090 on the Agulhas Ridge in the Atlantic sector of the Southern Ocean. This new sediment record shows evidence of a hitherto unknown late Eocene opal pulse. Lithological variations, compositional data, mass-accumulation rates of biogenic and lithogenic sediment constituents, grain-size distributions, geochemistry, and clay mineralogy are used to gain insights into mid-Cenozoic environmental changes and to explore the circumstances of the late Eocene opal pulse in terms of reorganizations in ocean circulation. The base of the section is composed of middle Eocene nannofossil oozes mixed with red clays enriched in authigenic clinoptilolite and smectite, deposited at low sedimentation rates (V 2 cm ka À 1). It indicates reduced terrigenous sediment input and moderate biological productivity during this preglacial warm climatic stage. The basal strata are overlain by an extended succession (100 m, 4 cm ka À 1) of biosiliceous oozes and muds, comprising the upper middle Eocene, the entire late Eocene, and the lowermost early Oligocene. The opal pulse occurred between 37.5 and 33.5 Ma and documents the development of upwelling cells along topographic highs, and the utilization of a marine nutrient-and silica reservoir established during the prelate Eocene through enhanced submarine hydrothermal activity and the introduction of terrigenous solutions from chemical weathering on adjacent continents. This palaeoceanographic overturn probably was initiated through the onset of increased meridional ocean circulation, caused by the diversion of the Indian equatorial current to the south. The opal pulse was accompanied by increased influxes of terrigenous detritus from southern African sources (illite), mediated by enhanced ocean particle advection in response to modified ocean circulation. The opal pulse ended because of frontal shifts to the south around the Eocene/Oligocene boundary, possibly in response to the opening of the Drake Passage and the incipient establishment of the Antarctic Circumpolar Current. Condensed sediments and a hiatus within the early Oligocene part of the section possibly point to an invigoration of the deep-reaching Antarctic Circumpolar Current. The mid-Oligocene to lower Miocene section on long time scale exhibits less pronounced lithological variations than the older section and points to relatively stable palaeoceanographic conditions after the dramatic changes in the late Eocene to early Oligocene.