Southern Ocean Sediments Research Articles

Biogenic opal in Southern Ocean sediments over the last 450,000 years: Implications for surface water chemistry and circulation

We present records of biogenic opal percentage and burial rate in 12 piston cores from the Atlantic and Indian sectors of the Southern Ocean. These records provide a detailed, quantitative description of changing patterns of opal deposition over the last 450 kyr. The striking regional coherence of these records suggests that dissolution in the deep sea and sediment pore waters does not obscure the surface productivity signal, and therefore these opal time series can be used in combination with other surface water tracers to make inferences about the chemistry and circulation of the Southern Ocean under different global climate conditions. Three broad depositional patterns can be distinguished. Northernmost records (39°–42°S latitude) are characterized by enhanced opal burial during glacial periods and strong 41 kyr periodicity. Records from cores just north of the present Antarctic Polar Front (46°–49°S) show even larger increases in opal burial rate during glacial intervals, but have variance concentrated in the 100 and 23 kyr bands. Southernmost records (51°–55°S) are completely out of phase with those to the north, with greatly reduced opal burial rates during glacial periods. Taken as a whole, the opal records show no evidence for the increased total Antarctic productivity predicted by recent geochemical models of atmospheric CO2 variability. The areal expansion of Southern Ocean sea ice over the present zone of high siliceous productivity provides one plausible explanation for the glacial‐interglacial opal patterns. The excess silica not taken up in this zone during glacial periods would contribute to greater nutrient availability and thus higher productivity in the subantarctic region. However, local circulation changes may act to modify this basic signal, possibly accounting for the observed differences in the opal variance spectra.

Dissolution and Preservation of Antarctic Diatoms and the Effect on Sediment Thanatocoenoses

AbstractComparison of Southern Ocean diatom populations from (i) surface ocean production, (ii) underlying Antarctic sediments, and (iii) laboratory dissolution experiments demonstrates that dissolution can account for the temporal and spatial variations in sedimentary diatom assemblages observed in Southern Ocean sediments. Increasing dissolution causes relative depletions in N. kerguelensis (K), enrichments in T. lentiginosa (L), and slight enrichments in E. antarctica (A). This reflects the relative susceptibility to dissolution of the three species that dominate Antarctic sediments. We have devised a preservation index for the Southern Ocean based on the ratio K/(K + L) to estimate relative extents of dissolution and applied it to natural assemblages. Holocene Southern Ocean sediments display increasing opal preservation toward higher latitudes, but south of the Antarctic Polar Front preservation decreases in the order: well preserved = SE Indian > S. Atlantic ∼ SW Indian > SE Pacific = poorly preserved. Dissolution also accounts for the pattern of diatom assemblages in the last glacial maximum (LGM) sediments of the Indian and Pacific sectors, but in the Atlantic, increased E. antarctica abundances at LGM must have resulted from an increase in surface ocean production of this species. Holocene and LGM diatoms in Atlantic and Pacific sector sediments are equally well preserved, but in the Indian sectors, Holocene sediments are better preserved than those of LGM age. Paleoceanographic and paleoclimatic transfer functions derived from factor analyses of variations in the sedimentary abundances of these three diatoms have ignored the effects of differential dissolution on thanatocoenosis and thus should be interpreted with caution.

Regional late Pliocene-early Pleistocene hiatuses of the Southern Ocean — Diatom evidence

The existence of regionally synchronous hiatuses in late Neogene sediments of the Southern Ocean has been questioned on sedimentological grounds, while their paleoclimatic interpretation has been questioned on theoretical grounds. We evaluated the evidence used to identify these hiatuses in Pliocene and lower Pleistocene sediments by determining quantitatively the occurrence of diatom stratigraphic markers in eleven Southern Ocean piston cores (three from the Pacific sector and eight from the Indian and Atlantic sectors). In the Pacific sector, some species have ranges similar to the published record but others show north-south diachrony. In the southeast Indian sector, some marker species have ranges at odds with the published magnetostratigraphy. In the Atlantic sector, there is wide disparity between our data and published results. In two of the cores, our placement of the Denticulopsis hustedtii datum differs with the published record by as much as 10 m. A third core contained a number of barren intervals and could not be zoned while a fourth core, previously zoned and dated with magneto- and biostratigraphy, contained too few diatoms for quantitative biostratigraphy. We conclude that the question of regional hiatuses in Southern Ocean sediments, while important, cannot be addressed until: (a) locally and regionally applicable biozonations are developed; and (b) magnetostratigraphic data are properly evaluated. Resolution of these problems are essential before we can successfully use hiatus data to recover the paleoceanographic and paleoclimatic history of the Southern Ocean.

Tertiary climatic evolution and vegetation history in the Southeast Indian Ocean region

The broad trends of climatic evolution in the Southeast Indian Ocean region during the Tertiary have become clearer with the accumulation of data from Legs 28 and 29 of the Deep Sea Drilling Project. The same events which are reflected in the deep-sea sediments clearly influenced the major continental areas of Australia and Antarctica and the vegetation which covered those regions. In this review, data relating to sea-surface temperatures, to land and sea positions, and to the extent of the Antarctic ice-cap, are used to derive climatic models for the Palaeocene to latest Miocene interval: the models are correlated with vegetation history deciphered from palynological data. For most of the Palaeocene, Australia and Antarctica were joined, and lay in high southern latitudes. Sea-surface temperatures were relatively high, and there is no evidence for Antarctic ice. The postulated atmospheric circulation patterns suggest a zone of westerly winds confined to 60–80°S, and, north of that, a wide zone of erratic circulation, with deep inland penetration of rain-bearing winds. Palynological evidence from Australia indicates rainforest in the southeast, with an extension to presently arid inland South Australia. In the Eocene, Australia and Antarctica were separated by a wide gulf; water temperatures were high — in the vicinity of 20°C on the Campbell Plateau — and evidence for Antarctic ice development is meagre. Atmospheric circulation was probably sluggish, with patterns similar to those of the Palaeocene. Vegetational data show the spread of rainforest communities across Australia from the east to the southwest, and locally, inland to central Australia. Vegetation zonation appears to have been minimal on that continent. In Antarctica, late Eocene vegetation was poorly diversified. For the Oligocene, evidence for a pronounced temperature drop is both isotopic and sedimentological, and includes the first record of ice-rafting near Antarctica. Intensification of atmospheric circulation seems likely. The vegetation records suggest lowered floristic diversity in Australia, and the persistence of vegetation, again of low diversity, in Antarctica into the late Oligocene. For the Miocene, the marine record suggests the development of a major Antarctic ice-cap; in consequence, atmospheric circulations continued to intensify. Continued northward drift of Australia meant increasing aridity in regions to the north and northwest; the south and east were watered by rain-bearing westerly systems and by embryonic Trade Winds. Vegetation data in Australia suggest rainforest cover in the southeast and on the east coast, and, in the middle Miocene, the development of grasslands in interfluvial areas in the central part of the continent. The latest Miocene was marked by an intense and sudden chilling which is clearly documented in Southern Ocean sediments. This event must have caused marked precipitation decrease in much of Australia, and may coincide with the disappearance of pollen of the Nothofagus brassi type from regions west of the Great Dividing Range, although age control of this event is poor.