Southern-sourced Water Research Articles

A δ13C record of Upper North Atlantic Deep Water during the past 2.6 million years

Benthic foraminiferal δ13C data from site 502 in the Caribbean Sea (sill depth ∼1800 m) indicate that throughout the past 2.6 m.y., glacial δ13C values in the middepth Atlantic were higher during glaciations than interglaciations. This is interpreted as indicating a greater proportion of Upper North Atlantic Deep Water (UNADW) relative to southern source waters during glaciations. The contribution of UNADW during interglaciations to the middepth Atlantic remained approximately constant, and the contribution during glaciations may have been as much as 10 % higher in the late Pleistocene than in the late Pliocene. This small increase is in striking contrast to the much larger decrease in glacial Lower North Atlantic Deep Water (LNADW) contribution relative to southern sources, from about 80% to about 20%, that occurred over the past 2.6 m.y. Glacial intensification over the past 2.6 m.y. was probably coupled with a decrease in northward heat transport by the upper limb of the North Atlantic circulation cell, as was previously suggested on the basis of a LNADW record alone. Late Pleistocene (1 Ma‐present) δ13C values in the Caribbean Sea were approximately 0.2‰ higher than they were from 2.6 to 2.0 Ma. The δ13C rise is not due to an increase in the mean ocean δ13C value, nor can it be entirely attributed to an increase in the proportion of high‐δ13C source waters. An increase in the δ13C value of the surface source waters must have contributed to the δ13C rise.

Cadmium, copper and nickel distributions at four stations in the eastern central and south Atlantic

Abstract Distributions of cadmium, copper and nickel at four stations in the eastern part of the Atlantic Ocean from 30 ° S to 34 ° N are described based on analytical results from three laboratories. The Cd and Ni profiles show nutrient-like distributions with concentrations in the deep waters increasing from north to south. Copper profiles all show gradual increases from surface to bottom with the highest concentrations occurring near bottom on the most northerly station. Variations in the deep-water Cd and Ni concentrations can be understood in terms of mixing of southern source waters with high concentrations with lower concentration northern source waters. The deep-water Cu distributions indicate a significant near-bottom source to the northern end of the section. Cadmium vs. phosphate relationships show features that result from both regeneration and mixing. Higher Cd:PO 4 ratios are seen in the southern source waters than in the northern waters, thus discounting the suggestion that the inflection in the global Cd:PO 4 relationship at PO 4 ≈ 1.3 μM originates in the southern ocean. Differential regeneration of Cd and PO 4 is seen through the equatorial oxygen minimum.

Pliocene Paleoceanography: Circulation and Oceanographic Changes Associated with the 2.4 Ma Glacial Event

High‐resolution δ18O records from the equatorial Pacific (site 503B), equatorial Atlantic (site 665A), and North Atlantic (site 606A) based on the benthic foraminifera Cibicidoides wuellerstorfi show the 2.4 Ma onset of major northern hemispheric glaciation to be a package of three events occurring at 2.39, 2.35, and 2.31 Ma in which a periodicity of about 40 kyr is evident. The amplitude of the signals at the three sites indicates that these events were 1/2 to 2/3 the size of the latest Quaternary glaciation and also indicates cooling of northern source bottom water by 2.7°–4.1°C relative to southern source water during glaciations. Carbon isotopes indicate that southern source waters were less oxygenated than in the Quaternary and that there was reduced production of northern source water during glacial intervals. The dominant presence of southern source water in the eastern basin of the equatorial Atlantic, regardless of climatic cycles, throughout the late Pliocene indicates a greater influence of these waters relative to northern source waters in the late Pliocene ocean.

Advective transport of clay minerals in the region of the Rio Grande Rise

Clay-mineral analyses (talc-normalized) were performed on a suite of 40 giant gravity core core-tops from the east flank of the Vema Channel that span the depth interval 1430–4565 m. In addition, three time-slice intervals, last glacial maximum (18,000 yrs B.P.), last interglacial (120,000 yrs B.P.), and penultimate glacial maximum (140,000 yrs B.P.) were examined in 25 of these cores. The clay mineral chlorite can be used to identify southern-source waters (AABW) in the Vema Channel. The mineralogical identification of the upper limit of AABW occurs at approximately 4000–4100 m, in agreement with estimates based upon displaced Antarctic diatoms and modern oceanographic data. Time-slice reconstructions of chlorite abundance suggest AABW flow intensity was slightly reduced during glacial relative to interglacial intervals. The clay mineral kaolinite reaches maximum concentration at mid depth (approximately 3500 m) on the Rio Grande Rise but does not appear to be advectively transported to the Rise by southward-flowing NADW, rather kaolinite-rich sediments appear to be transported eastward from the São Paulo Plateau to the Rio Grande Rise along isopycnals. This process is estimated to contribute approximately 13% of the fine-grained lithogenic sediments reaching the Rio Grande Rise at approximately 3500 m.

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Census counts of benthic foraminifera were studied from the SW Iberian Margin to reconstruct past changes in deep-water hydrography across Terminations I and II. Detailed benthic faunal data (> 125 μm size-fraction) allow us to evaluate the limitations imposed by taphonomic processes and restricted size-fractions. The comparison of recent (mudline) and fossil assemblages at IODP Site U1385 indicates the quick post-mortem disintegration of shells of astrorhizoid taxa (~ 80% of the present-day fauna), resulting in impoverished fossil assemblages. While the application of quantitative proxy methods is problematic under these circumstances, the fossil assemblages can still provide a qualitative palaeoenvironmental signal that, while most fully expressed in the 125–212 μm size-fraction, is nonetheless also expressed to some degree in the > 212 μm size-fraction.Variations in the benthic foraminiferal assemblages reveal information about changing organic matter supply, deep-water oxygenation and temperature. MIS 2 is generally characterized by an elevated trophic state and variable oxic conditions, with oxygenation minima culminating in the Younger Dryas (YD) and Heinrich Stadials (HS) 1, 2 and 3. Low oxic conditions coincide with decreased water-temperature and lower benthic δ13C, pointing to the strong influence of a southern sourced water-mass during these periods. HS 1 is the most extreme of these intervals, providing further evidence for a severe temporary reduction or even shutdown of AMOC. With the inception of MIS 1, organic matter supply reduced and a better ventilated deep-water environment bathed by NEADW is established.For Termination II, clear indications of southern-sourced water are limited to the early phase of HS 11. During the latter part of HS 11, the deep-water environment seems to be determined by strongly increased supply of organic matter, potentially explaining the decoupling of benthic δ13C and Mg/Ca records of earlier studies as a phytodetritus effect on the carbon isotope signal. However, the presence of a warm, nutrient-rich and poorly oxygenated water-mass cannot be ruled out. With the inception of interglacial MIS 5e trophic conditions are reduced and ventilation by NEADW increases.

Geographic variations in the relationship between silicic acid and nitrate in the South Pacific Ocean

Scatter plots of silicic acid vs nitrate concentration from stations in the South Pacific Ocean and the Pacific Sector of the Southern Ocean divide the area into zones of nutrient balance, excess silicic acid, or excess nitrate. The boundaries of the zones are similar to published surface-water mass maps. A T-S analysis of stations divided on the basis of nutrient character show that: (1) South Pacific Central Water is in nutrient balance; (2) Subantarctic Surface Water has actual excess nitrate and Antarctic Circumpolar Water (Surface) has potential excess nitrate; and (3) Ross Sea Water exhibits potential excess silicic acid. The actual nitrate excess in Subantarctic Surface Water is attributed to the circumpolar character of the water mass, to the extensive diatom growth in southern source water, and to the different mechanisms of regeneration of silicic acid and nitrate. The nutrient character of this water mass probably effects food web dynamics in its resident area and in coastal areas on which it impinges. The potential silicic acid excess of Ross Sea Water is attributed to the upwelling of silicic acid-enriched deep water at the Antarctic Divergence (≈ 70°S), which probably contributes to excess silicic acid in the North Pacific Ocean through Antarctic Bottom Water formation.