No abstract

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

No abstract

This paper summarizes the biostratigraphy and magnetostratigraphy of the 11 sites drilled on the Kerguelen Plateau and in Prydz Bay, Antarctica, during ODP Leg 119. Excellent magnetobiochronologic reference sections were obtained at deep-water Sites 745 and 746 (0-10 Ma) and at intermediate depth Site 744 (0-39 Ma) on the southern Kerguelen Plateau. Site 738, an intermediate depth companion site for Site 744, contains a nearly complete lowermost Oligocene to Turonian carbonate section including a continuous sequence across the Cretaceous/Tertiary boundary. Northern Kerguelen Sites 736 and 737 (ca. 600 m water depth) constitute a composite middle Eocene to Quaternary reference section near the present-day Antarctic Polar Front. Biostratigraphic control is limited in Prydz Bay Sites 739-743. Glacial sequences cored on the continental shelf at Sites 739 and 742 appear to form a composite record, possibly from the uppermost middle Eocene to the Quaternary; the entire upper Oligocene and most of the Miocene, however, are removed at an unconformity. Preglacial sediments at Site 741 contain Early Cretaceous pollen and spores, but the red beds cored at Site 740 are unfossiliferous. Poorly-fossiliferous glacial sediments of probable Quaternary age were sampled on the upper slope at Site 743. A magnetobiochronologic time scale is presented for the Late Cretaceous and Cenozoic of the Southern Ocean based on previous studies and the results of Leg 119 studies.

Samples were examined for diatoms from 22 holes at 11 sites cored by ODP Leg 119 on the Kerguelen Plateau and in Prydz Bay, East Antarctica. Diatoms were observed in Oligocene through Holocene sediments recovered from the Kerguelen Plateau. The diatom flora from the Kerguelen Plateau is characterized by species such as Azpeitia oligocenica, Rocella gelida, Rocella vigilans, and Synedra jouseana in the Oligocene and Crucidenticula nicobarica, Denticulopsis hustedtii, Nitzschia miocenica, and Thalassiosira miocenica in the Miocene. This somewhat cosmopolitan assemblage gives way to a Pliocene and Holocene assemblage characterized by species such as Nitzschia kerguelensis, Thalassiosira inura, and Thalassiosira torokina, which are endemic to the Southern Ocean region. Samples examined from Prydz Bay are generally devoid of diatoms. The exception is Site 739, where diatoms occur sporadically in lower Oligocene and upper Miocene through Quaternary sediments.The Leg 119 diatom biostratigraphic results allow the development of a stratigraphic framework for the Indian sector of the Southern Ocean. This diatom zonation integrates diatom zonations developed previously for other sectors of the Southern Ocean. The zonation proposed here is based on biostratigraphic events of both geographically widespread and endemic species calibrated to the paleomagnetic stratigraphy. As such, this zonation has application throughout the Southern Ocean and allows correlation from the southern high latitudes to the low latitudes.

No abstract

Two major goals of Leg 108 were to investigate late Cenozoic changes in (1) North African aridity and (2) atmospheric circulation over the equatorial Atlantic and Sahelian/Saharan Africa. Several high-resolution records from Ocean Drilling Program Leg 108 are pertinent to these problems. Dust fluxes from Africa to the Atlantic were low during the final 3 m.y. of the Miocene and then increased markedly during the Pliocene and Pleistocene. The increasing Pliocene-Pleistocene dust fluxes suggest major aridification of North Africa, possibly accompanied by an increase in the amplitude of aridity/humidity cycles. Other evidence from the northwest African margin (influxes of fluvial clay, terrestrial carbon, freshwater diatoms, and pollen) also suggests increasing aridity and larger oscillations during the Pliocene and Pleistocene, along with increased intensity of coastal trade winds. Because prominent changes in long-term dust fluxes preceded Northern Hemisphere glaciation by 1.5 Ma, Northern Hemisphere ice sheets were not the major factor in the evolution of African climate, in agreement with late Pleistocene evidence at orbital time scales. The apparent synchroneity of several major long-term changes in climate over Africa and the equatorial Atlantic with changes in the Southern Ocean and South Atlantic suggests long-term linkage in the responses of these two regions, again similar to late Pleistocene linkages at orbital time scales. The ultimate source of forcing of these changes at tectonic time scales is not fully resolved. The Messinian closing and abrupt reopening of the Mediterranean left no obvious imprint on signals of African dust flux. One plausible source of forcing is large-scale tectonic uplift, which occurred at unusually rapid rates during the latest Cenozoic in Southeast Asia (Tibet and the Himalayas), East Africa, and South America (the Andes and Altiplano). Modeling experiments show that uplift causes large-scale rearrangements of atmospheric circulation, including the strength and position of the upper tropospheric jet streams and the lower tropospheric high-and low-pressure cells that control surface winds and moisture balances.

The equatorial Pacific experienced significant changes in productivity and microfossil assemblage since 16 million years ago (Ma). We compiled a record of those changes from IODP Site U1338 using a reconnaissance of diatom assemblages and high-resolution XRF-scan chemical profiles (1-2 kyr spacing). Productivity and CaCO3 dissolution intervals are defined by sediment component ratios, in particular opal/clay opal/BaSO4 and CaCO3/BaSO4 or 23 CaCO3/clay. There are large abrupt changes in export production in the Miocene, especially compared to the Pleistocene and Pliocene and average levels of export production are higher throughout the Miocene compared to the Pliocene and Pleistocene. Using diatom assemblages and bulk sediment composition, the U1338 sediment record is divided into several distinct deposition regimes: (1) a Middle Miocene regime >13.2 Ma marked by low diatom numbers and a few short-lived productivity intervals, (2) a Carbonate Crash regime distinguished by an older substage (13.2-10.2 Ma) with relatively high abundances of productivity-related diatoms and common productivity-related depositional intervals interspersed with CaCO3 dissolution intervals and a younger carbonate crash substage (10.2-8.0 Ma) with weaker brief productivity 32 intervals, high CaCO3 dissolution, and moderately high numbers of upwelling diatom species, (3) the Biogenic Bloom regime (8.0-4.5 Ma) marked by extended periods of high opal and CaCO3 deposition with a maximum between 7.0 and 6.4 Ma, and (4) a Pliocene-Modern regime 35 (4.5-0 Ma) with lower productivity and high cyclic CaCO3 dissolution. Changes in production and dissolution result from reorganizations of nutrient supply to the equatorial Pacific, not by higher wind-driven upwelling. We propose that nutrients were more accessible in the Miocene because of a combination of factors including higher organic matter degradation in the upper water column and deep-intermediate nutrient pathways that efficiently recycled nutrients to 3 upwelling zones. Closure of the Central American Seaway was likely a prime cause of the Biogenic Bloom.