Andrea N A Volbers

Present water mass calcium carbonate corrosiveness in the eastern South Atlantic inferred from ultrastructural breakdown of Globigerina bulloides in surface sediments

Abstract The Atlantic is regarded as a huge carbonate depocenter due to an on average deep calcite lysocline. However, calculations and models that attribute the calcite lysocline to the critical undersaturation depth (hydrographic or chemical lysocline) and not to the depth at which significant calcium carbonate dissolution is observed (sedimentary calcite lysocline) strongly overestimate the preservation potential of calcareous deep-sea sediments. Significant calcium carbonate dissolution is expected to begin firstly below 5000 m in the deep Guinea and Angola Basin and below 4400 m in the Cape Basin. Our study that is based on different calcium carbonate dissolution stages of the planktic foraminifera Globigerina bulloides clearly shows that it starts between 400 and 1600 m shallower depending on the different hydrographic settings of the South Atlantic Ocean. In particular, coastal areas are severely affected by increased supply of organic matter and the resultant production of metabolic CO2 which seems to create microenvironments favorable for dissolution of calcite well above the hydrographic lysocline.

Calcium carbonate corrosiveness in the South Atlantic during the Last Glacial Maximum as inferred from changes in the preservation of Globigerina bulloides: A proxy to determine deep-water circulation patterns?

Abstract The modern Atlantic Ocean, dominated by the interactions of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW), plays a key role in redistributing heat from the Southern to the Northern Hemisphere. In order to reconstruct the evolution of the relative importance of these two water masses, the NADW/AABW transition, reflected by the calcite lysocline, was investigated by the Globigerina bulloides dissolution index (BDX′). The depth level of the Late Glacial Maximum (LGM) calcite lysocline was elevated by several hundred metres, indicating a more corrosive water mass present at modern NADW level. Overall, the small range of BDX′ data and the gradual decrease in preservation below the calcite lysocline point to a less stratified Atlantic Ocean during the LGM. Similar preservation patterns in the West and East Atlantic demonstrate that the modern west–east asymmetry did not exist due to an expansion of southern deep waters compensating for the decrease in NADW formation.

Late Quaternary variations in calcium carbonate preservation of deep-sea sediments in the northern Cape Basin: results from a multiproxy approach

In this study, we test various parameters in deep-sea sediments (bulk sediment parameters and changes in microfossil abundances and preservation character) which are generally accepted as indicators of calcium carbonate dissolution. We investigate sediment material from station GeoB 1710-3 in the northern Cape Basin (eastern South Atlantic), 280 km away from the Namibian coast, well outside today’s coastal upwelling. As northern Benguela upwelling cells were displaced westward and periodically preceded the core location during the past 245 kyr (Volbers et al., submitted), GeoB 1710-3 sediments reflect these changes in upwelling productivity. Results of the most commonly used calcium carbonate dissolution proxies do not only monitor dissolution within these calcareous sediments but also reflect changes in upwelling intensity. Accordingly, these conventional proxy parameters misrepresent, to some extent, the extent of calcium carbonate dissolution. These results were verified by an independent dissolution proxy, the Globigerina bulloides dissolution index (BDXP) (Volbers and Henrich, submitted). The BDXP is based on scanning electronic microscope ultrastructural investigation of planktonic foraminiferal tests and indicates persistent good carbonate preservation throughout the past 245 kyr, with the exception of one pronounced dissolution event at early oxygen isotopic stage (OIS) 6. The early OIS 6 is characterized by calcium carbonate contents, sand contents, and planktonic foraminiferal concentrations all at their lowest levels for the last 245 kyr. At the same time, the ratio of radiolarian to planktonic foraminiferal abundances and the ratio of benthic to planktonic foraminiferal tests are strongly increased, as are the rain ratio, the fragmentation index, and the BDXP. The sedimentary calcite lysocline rose above the core position and GeoB 1710-3 sediments were heavily altered, as attested to by the unusual accumulation of pellets, aggregates, sponge spicules, radiolaria, benthic foraminifera, and planktonic foraminiferal assemblages. Solely the early OIS 6 dissolution event altered the coarse fraction intensely, and is therefore reflected by all conventional calcium carbonate preservation proxies and the BDXP. We attribute the more than 1000 m rise of the sedimentary calcite lysocline to the combination of two processes: (a) a prominent change in the deep-water mass distribution within the South Atlantic and (b) intense degradation of organic material within the sediment (preserved as maximum total organic carbon content) creating microenvironments favorable for calcium carbonate

Carbonate preservation in deep and intermediate water masses in the South Atlantic: evaluation and geological record (a review)

Evaluation of conventional dissolution proxies in South Atlantic surface sediments revealed broad applicability only in far offshore, rather oligotrophic regimes in the western basins. In contrast, they fail or produce misleading and incorrect results in the more productive eastern South Atlantic basins, due to the combined effects of variable dilution by non-carbonate material and fluctuating ecological conditions. Much more promising are the results from new dissolution proxies on the planktic foraminifer Globigerina bulloides (BDX’) and the pteropod Limacina inflate (LDX) which were calibrated with carbonate saturation as indicated by GEOSECS data. In the western South Atlantic, the sedimentary calcite lysocline is encountered by the BDX’ at the transition between AABW and LNADW. However, it rises up into the LNADW close to the equator due to additional supralysoclinal dissolution. In the eastern South Atlantic basins, supralysoclinal dissolution results in an elevation of the sedimentary calcite lysocline of several hundred metres to a maximum of 1600 m as compared to the position of the hydrographic lysocline, with aragonite preservation in the eastern South Atlantic being even poorer. At most sites investigated, the surface sediments are void of pteropods and thus LDX failure is indicated. However, in the western South Atlantic the LDX displays a double lysocline for aragonite, the upper lysocline at a water depth of 750 m and the lower at 2500 m. Aragonite and calcite preservation profiles indicate much weaker stratification of the water during the LGM. With 3200 m, the position of the calcite lysocline is encountered at the same level in the southern parts of the eastern and western basins dropping to 4000 m near the equator. Along the western continental margin no indication for aragonite-corrosive glacial AAIW was found, providing clear evidence for a strengthened GNAIW flow along the Brazil margin. The long-term history of carbonate dissolution in the equatorial Atlantic was reconstructed by a multiproxy approach combining benthic foraminifer stable isotopes and new proxies from silt analysis. For the first time, this allows a reconstruction of the chemical (nutrient content, carbonate corrosiveness) and physical (bottom current strength) properties of deep and intermediate water masses. The terrigenous silt records of ODP Site 927 at the Ceara Rise show rapid shifts from low to very high bottom-currents speeds for nearly all the isotopic transitions in the Brunhes epoch, indicating subsequent phases of shutdown and rapid reinstatement of LNADW circulation. A drastic reduction of glacial bottom-current strength at Site 927 is inferred after 2.75 Ma, synchronous with the first occurrence of larger continental ice shields and with a drastic decrease in deep convection in the Norwegian-Greenland Sea. After the mid-Pleistocene climate transition, progressively weaker bottom currents and poorer carbonate preservation during glacials indicate a progressive reduction of LNADW from the Late Pliocene to Pleistocene. On the contrary, an opposite trend with progressive improvement of preservation during glacials from Late Pliocene to the Pleistocene is observed in the Caribbean at Site 999. This indicates a contemporaneous progressive increase in the contribution of UNADW to the Atlantic in glacial periods. Altogether, a progressive weakening of the circulation in the LNADW loop and a contemporaneous strengthening of the UNADW loop are evident since the mid Pleistocene transition.

Contribution of Calcareous Plankton Groups to the Carbonate Budget of South Atlantic Surface Sediments

A total of more than 400 surface sediment samples from the equatorial, central and subpolar South Atlantic Ocean were investigated for their carbonate content as well as for the carbonate contribution from various calcareous plankton groups. The modem pattern of marine carbonate production is exemplified by comparing two sediment traps located in different domains of the South Atlantic. In addition, this paper presents new carbonate calculations for the content of coccoliths, calcareous dinocysts, planktic foraminifera, and pteropods in surface sediments. In general, carbonate input of the different organism groups is highly variable although dominated by both planktic foraminifera and coccolithophorids. Whereas coccolith carbonate dominates the oligotrophic gyres of the South Atlantic, carbonate derived from planktic foraminifera is much more important in more fertile, mesotrophic areas, such as the equatorial divergence zone. In contrast, calcareous dinocysts only supply a minor proportion of calcium carbonate to the sediments. The aragonite content, mainly derived from pteropod shells, is of regional importance at the continental margin of the western South Atlantic. Here, aragonite contents of up to 50 wt-% of the total sediments were measured. Carbonate dissolution has a major effect below the lysocline depth, but also in highly productive areas (supralysoclinal dissolution). Foraminiferal carbonate is much more affected by dissolution than either coccolith or calcareous dinocyst carbonate. Preservation of pteropod shells is restricted to relatively shallow parts of the ocean distant from continental margins, as aragonite is much more susceptible to dissolution than calcite. As a result, the maximum aragonite content is observed at an intermediate depth, i.e. between 2000 to 3000m.

Coccolithophorid and Dinoflagellate Synecology in the South and Equatorial Atlantic: Improving the Paleoecological Significance of Phytoplanktonic Microfossils

Individual planktonic microfossil species, or assemblage groups of different species, are often used to, qualitatively and/or quantitatively, reconstruct past (sub)surface-water conditions of the world’s oceans and seas. Until now, little information has been available on the surface sediment distribution patterns and paleoenvironmental reconstruction potential of coccolith, calcareous dinoflagellate cyst and organic-walled dinoflagellate cyst assemblages of the South and equatorial Atlantic, especially at the species level. This paper (i) summarizes the distributions of these three phytoplanktonic microfossil groups in numerous Atlantic surface sediments from 20°N–50°S and 30°E–65°W and determines their relationship with the physicochemical and trophic conditions of the overlying (sub)surface-waters, and (ii) determines the synecology of the three phytoplankton groups by carrying out statistical analyses (i.e. detrended and canonical correspondence analyses) on all groups simultaneously. Ecological relationships are additionally strengthened by statistically comparing the distribution patterns ofthe phytoplankton groups with those of planktonic foraminifera (Pflaumann et al. 1996; Niebler et al. 1998), as the ecological preferences of the latter are much better known. Many of the analyzed phytoplanktonic microfossil species or groups of species in the surface sediments do show restricted distributions which primarily reflect the environmental conditions of the upper water masses above them (e.g. sea-surface temperature, productivity, stratification). The acquired ‘reference’ data sets are large and diverse enough to allow future development of transfer functions for the reconstruction of past surface-water conditions, and show that there is still an enormous paleoenvironmental reconstruction potential concealed in many fossil coccolith and dinoflagellate cyst assemblages.