Aragonite preservation in late Quaternary sediment cores on the Brazilian Continental Slope: implications for intermediate water circulation

Abstract

We present late Quaternary records of aragonite preservation determined for sediment cores recovered on the Brazilian Continental Slope (1790±2585 m water depth) where North Atlantic Deep Water (NADW) dominates at present. We have used various indirect dissolution proxies (carbonate content, aragonite/calcite contents, and sand percentages) as well as gastropodal abundances and fragmentation of Limacina inflata to determine the state of aragonite preservation. In addition, microscopic investigations of the dissolution susceptibility of three Limacina species yielded the Limacina Dissolution Index which correlates well with most of the other proxies. Excellent preservation of aragonite was found in the Holocene section, whereas aragonite dissolution gradually increases downcore. This general pattern is attributed to an overall increase in aragonite corrosiveness of pore waters. Overprinted on this early diagenetic trend are high-frequency fluctuations of aragonite preservation, which may be related to climatically induced variations of intermediate water masses.

Coupling of millennial‐scale changes in sea surface temperature and precipitation off northeastern Brazil with high‐latitude climate shifts during the last glacial period

“…Calcium mainly reflects the biogenic carbonate content whereas titanium, which is related to siliciclastic sediment components, varies directly with the terrigenous fraction of the sediment. Changes in glacial carbonate production and dissolution between 2000 and 3000 m water depth in the western tropical Atlantic were relatively minor [Rühlemann et al, 1996;Gerhardt et al, 2000]. Therefore maxima in Ti/Ca ratios characterize periods when precipitation in the Brazilian hinterland abruptly increased and erosion and fluvial transport of terrigenous matter to the shelf and continental slope were strongly enhanced ].…”

Methods

“…Pteropod assemblages are found to be useful for inferring oceanographic circulation and ventilation conditions at intermediate depth. Changes in abundance variation and preservation condition of their assemblages are related to hydrographic parameters like salinity, temperature, and oxygen conditions, as well as the aragonite saturation state of deep water (e.g., Almogi-Labin et al, 1986;Gerhardt et al, 2000;Singh et al, 2005). They are frequent north of the Azores Front (Schiebel et al, 2002), which during the Last Glacial Maximum extended into the Gulf of Cádiz (Rogerson et al, 2004).…”

Pteropods from the Caribbean Sea: variations in calcification as an indicator of past ocean carbonate saturation

“…Chen (1968) suggests that their widespread occurrence was controlled by Late Pleistocene climate changes. This latest Pleistocene occurrence of abundant pteropods has also been recorded in the Andaman Sea (Sijinkumar et al, 2010), in the Red Sea (Almogi-Labin et al, 1991), offshore Florida (Gardulski et al, 1990), on the western flank of the Great Bahama Bank (Eberli et al, 1997;Messenger et al, 2010), on the Brazilian Slope (Gerhardt et al, 2000), in the Caribbean Sea (Haddad and Droxler, 1996), off-shore Somalia and in the South China Sea (Wang et al, 1997). In the cores from the South China Sea and the Caribbean Sea, the concentrations at ∼20 000 years BP and 150 000 years BP are both recorded, clearly demonstrating that this enhanced preservation of aragonitic fossils is of global significance and not the result of local variations in water chemistry (Peterson and Cofer-Shabica, 1987;Peterson, 1990, Broecker andClark, 2002;Sepulcre et al, 2009).…”

“…Only whole specimens that retained their protoconch and protoconch fragments were counted. Determination of the calcification of the pteropod shells was made using the Limacina Dissolution Index (LDX) which was devised by Gerhardt et al (2000) and published as a scale by Gerhardt and Henrich (2001). This method involves the semi-quantitative analysis of the surface of the pteropod shell on a scale of 0 to 5; 0 being a pristine, transparent, lustrous shell with a smooth surface and 5 being an opaque, white and completely lustreless shell with additional damage.…”