Shell preservation of Limacina inflata in surface sediments of the Central and South Atlantic

Over 300 surface sediment samples from the Central and South Atlantic Ocean and the Caribbean Sea were investigated for the preservation state of the aragonitic test of Limacina inflata. Results are displayed in spatial distribution maps and are plotted against cross-sections of vertical water mass configurations, illustrating the relationship between preservation state, saturation state of the overlying waters, and overall water mass distribution. The microscopic investigation of L. inflata (adults) yielded the Limacina dissolution index (LDX), and revealed three regional dissolution patterns. In the western Atlantic Ocean, sedimentary preservation states correspond to saturation states in the overlying waters. Poor preservation is found within intermediate water masses of southern origin (i.e. Antarctic intermediate water (AAIW), upper circumpolar water (UCDW)), which are distinctly aragonite-corrosive, whereas good preservation is observed within the surface waters above and within the upper North Atlantic deep water (UNADW) beneath the AAIW. In the eastern Atlantic Ocean, in particular along the African continental margin, the LDX fails in most cases (i.e. less than 10 tests of L. inflata per sample were found). This is most probably due to extensive "metabolic" aragonite dissolution at the sediment-water interface combined with a reduced abundance of L. inflata in the surface waters. In the Caribbean Sea, a more complex preservation pattern is observed because of the interaction between different water masses, which invade the Caribbean basins through several channels, and varying input of bank-derived fine aragonite and magnesian calcite material. The solubility of aragonite increases with increasing pressure, but aragonite dissolution in the sediments does not simply increase with water depth. Worse preservation is found in intermediate water depths following an S-shaped curve. As a result, two aragonite lysoclines are observed, one above the other. In four depth transects, we show that the western Atlantic and Caribbean LDX records resemble surficial calcium carbonate data and delta13C and carbonate ion concentration profiles in the water column. Moreover, preservation of L. inflata within AAIW and UCDW improves significantly to the north, whereas carbonate corrosiveness diminishes due to increased mixing of AAIW and UNADW. The close relationship between LDX values and aragonite contents in the sediments shows much promise for the quantification of the aragonite loss under the influence of different water masses. LDX failure and uncertainties may be attributed to (1) aragonite dissolution due to bottom water corrosiveness, (2) aragonite dissolution due to additional CO2 release into the bottom water by the degradation of organic matter based on an enhanced supply of organic matter into the sediment, (3) variations in the distribution of L. inflata and hence a lack of supply into the sediment, (4) dilution of the sediments and hence a lack of tests of L. inflata, or (5) redeposition of sediment particles.

(Table 2) Ice rafted debris abundance and accumulation rate, sedimentation rate and dry bulk density of bottom sediments from the Sea of Okhotsk

Oxygen isotope records, radiocarbon AMS data, carbonate and opal stratigraphy, sediment magnetic susceptibility, tephrachronology, and paleontological results were used to obtain detailed sediment stratigraphy and an age model for the studied cores. For studying sea-ice sedimentation an analysis of lithogenic grain number in >0.15 mm grain size fraction of bottom sediments was carried out. For quantitative estimation of intensity ice-rafting debris sedimentation number of IRD particles per sq cm per ka was calculated. Obtained results allowed to plot IRD AR distribution for the first oxygen isotope stage (0-12.5 14C ka, 14C) and for the second stage (12.5-24 14C ka). The first stage was subdivided into the latest deglaciation and the beginning of Holocene (6-12.5 14C ka) (transitive period), when the sea level was changing significantly, and the second part of Holocene (0-6 14C ka), when climate conditions and the sea level were similar to modern estimates. Data clearly show strong increase in ice formation in the glacial Sea of Okhotsk and its extent in the middle part of the sea. Average annual duration of ice coverage during glaciation was longer than that for interglaciation. However the sea ice cover was not continuous all the year round and disappeared in summer time except the far northwestern part of the sea.

Geochemistry of surface sediment samples from the South Atlantic Ocean

The aim of the present study is an evaluation of the applicability of biogenic barium as a proxy for productivity. For this purpose, 190 surface sediment samples from the South Atlantic Ocean were analysed for their barium and aluminium concentrations. Biogenic barium is estimated by subtracting the calculated terrigenous barium (obtained from the terrigenous Ba/Al ratio and the amount of Al in the sample) from the total Ba content in the sample. Based on the accumulation rates of biogenic barium, export production is estimated using three different algorithms proposed by [Paleoceanography 7 (1992) 163, doi:10.1029/92PA00181; Global Biogeochem. Cycles 9 (1995) 289, doi:10.1029/95GB00021; Geomar. Report 38 (1995) 105]. Primary productivity was calculated from these different export productions and compared with measurements of recent primary productivity in the overlying surface waters. Only the primary productions calculated on the basis of the algorithm of [Paleoceanography 7 (1992) 163, doi:10.1029/92PA00181] yield productivity values comparable to those existing in ocean surface waters. This study further reveals that it is not sufficient to use a constant, generally applicable organic carbon/biogenic barium ratio, as is postulated by [Global Biogeochem. Cycles 9 (1995) 289, doi:10.1029/95GB00021]. This ratio has to be assessed regionally. For the sediments of the Cape Basin in the eastern South Atlantic Ocean, a new algorithm is developed which gives plausible primary productivities for the overlying surface waters.

Global compilation of benthic data sets I

Approaches to quantify the organic carbon accumulation on a global scale generally do not consider the small-scale variability of sedimentary and oceanographic boundary conditions along continental margins. In this study, we present a new approach to regionalize the total organic carbon (TOC) content in surface sediments (<5 cm sediment depth). It is based on a compilation of more than 5500 single measurements from various sources. Global TOC distribution was determined by the application of a combined qualitative and quantitative-geostatistical method. Overall, 33 benthic TOC-based provinces were defined and used to process the global distribution pattern of the TOC content in surface sediments in a 1°x1° grid resolution. Regional dependencies of data points within each single province are expressed by modeled semi-variograms. Measured and estimated TOC values show good correlation, emphasizing the reasonable applicability of the method. The accumulation of organic carbon in marine surface sediments is a key parameter in the control of mineralization processes and the material exchange between the sediment and the ocean water. Our approach will help to improve global budgets of nutrient and carbon cycles.

Compilation of planktonic foraminiferal census counts from the Southern Hemisphere Oceans

We present an improved database of planktonic foraminiferal census counts from the Southern Hemisphere Oceans (SHO) from 15°S to 64°S. The SHO database combines 3 existing databases. Using this SHO database, we investigated dissolution biases that might affect faunal census counts. We suggest a depth/[DCO3]2- threshold of ~3800 m/[DCO3]2- = ~-10 to -5 µmol/kg for the Pacific and Indian Oceans, and ~4000 m/[DCO3]2- = ~0 to 10 µmol/kg for the Atlantic Ocean, under which core-top assemblages can be affected by dissolution and are less reliable for paleo-sea surface temperature (SST) reconstructions. We removed all core-tops beyond these thresholds from the SHO database. This database has 598 core-tops and is able to reconstruct past SST variations from 2° to 25.5°C, with a root mean square error of 1.00°C, for annual temperatures. To inspect dissolution affects SST reconstruction quality, we tested the data base with two "leave-one-out" tests, with and without the deep core-tops. We used this database to reconstruct Summer SST (SSST) over the last 20 ka, using the Modern Analog Technique method, on the Southeast Pacific core MD07-3100. This was compared to the SSST reconstructed using the 3 databases used to compile the SHO database. Thus showing that the reconstruction using the SHO database is more reliable, as its dissimilarity values are the lowest. The most important aspect here is the importance of a bias-free, geographic-rich, database. We leave this dataset open-ended to future additions; the new core-tops must be carefully selected, with their chronological frameworks, and evidence of dissolution assessed.

Glacial North Atlantic: Sea surface conditions reconstructed by GLAMAP 2000

The response of the tropical ocean to global climate change and the extent of sea ice in the glacial nordic seas belong to the great controversies in paleoclimatology. Our new reconstruction of peak glacial sea surface temperatures (SSTs) in the Atlantic is based on census counts of planktic foraminifera, using the Maximum Similarity Technique Version 28 (SIMMAX-28) modern analog technique with 947 modern analog samples and 119 well-dated sediment cores. Our study compares two slightly different scenarios of the Last Glacial Maximum (LGM), the Environmental Processes of the Ice Age: Land, Oceans, Glaciers (EPILOG), and Glacial Atlantic Ocean Mapping (GLAMAP 2000) time slices. The comparison shows that the maximum LGM cooling in the Southern Hemisphere slightly preceeded that in the north. In both time slices sea ice was restricted to the north western margin of the nordic seas during glacial northern summer, while the central and eastern parts were ice-free. During northern glacial winter, sea ice advanced to the south of Iceland and Faeroe. In the central northern North Atlantic an anticyclonic gyre formed between 45° and 60°N, with a cool water mass centered west of Ireland, where glacial cooling reached a maximum of >12°C. In the subtropical ocean gyres the new reconstruction supports the glacial-to-interglacial stability of SST as shown by CLIMAP Project Members (CLIMAP) [1981]. The zonal belt of minimum SST seasonality between 2° and 6°N suggests that the LGM caloric equator occupied the same latitude as today. In contrast to the CLIMAP reconstruction, the glacial cooling of the tropical east Atlantic upwelling belt reached up to 6°-8°C during Northern Hemisphere summer. Differences between these SIMMAX-based and published U37[k]- and Mg/Ca-based equatorial SST records are ascribed to strong SST seasonalities and SST signals that were produced by different planktic species groups during different seasons.

Distribution of planktic foraminifera in surface sediments of the Atlantic Ocean

We present a data set of 738 planktonic foraminiferal species counts from sediment surface samples of the eastern North Atlantic and the South Atlantic between 87°N and 40°S, 35°E and 60°W including published Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) data. These species counts are linked to Levitus's [1982] modern water temperature data for the four caloric seasons, four depth ranges (0, 30, 50, and 75 m), and the combined means of those depth ranges. The relation between planktonic foraminiferal assemblages and sea surface temperature (SST) data is estimated using the newly developed SIMMAX technique, which is an acronym for a modern analog technique (MAT) with a similarity index, based on (1) the scalar product of the normalized faunal percentages and (2) a weighting procedure of the modern analog's SSTs according to the inverse geographical distances of the most similar samples. Compared to the classical CLIMAP transfer technique and conventional MAT techniques, SIMMAX provides a more confident reconstruction of paleo-SSTs (correlation coefficient is 0.994 for the caloric winter and 0.993 for caloric summer). The standard deviation of the residuals is 0.90°C for caloric winter and 0.96°C for caloric summer at 0-m water depth. The SST estimates reach optimum stability (standard deviation of the residuals is 0.88°C) at the average 0- to 75-m water depth. Our extensive database provides SST estimates over a range of -1.4 to 27.2°C for caloric winter and 0.4 to 28.6°C for caloric summer, allowing SST estimates which are especially valuable for the high-latitude Atlantic during glacial times.

Diatom abundance in surface sediments of the northern North Pacific

In order to map the modern distribution of diatoms and to establish a reliable reference data set for paleoenvironmental reconstruction in the northern North Pacific, a new data set including the relative abundance of diatom species preserved in a total of 422 surface sediments was generated, which covers a broad range of environmental variables characteristic of the subarctic North Pacific, the Sea of Okhotsk and the Bering Sea between 30° and 70°N. The biogeographic distribution patterns as well as the preferences in sea surface temperature of 38 diatom species and species groups are documented. A Q-mode factor analysis yields a three-factor model representing assemblages associated with the Arctic, Subarctic and Subtropical water mass, indicating a close relationship between the diatom composition and the sea surface temperatures. The relative abundance pattern of 38 diatom species and species groups was statistically compared with nine environmental variables, i.e. the summer sea surface temperature and salinity, annual surface nutrient concentration (nitrate, phosphate, silicate), summer and winter mixed layer depth and summer and winter sea ice concentrations. Canonical Correspondence Analysis (CCA) indicates 32 species and species groups have strong correspondence with the pattern of summer sea surface temperature. In addition, the total diatom flux data compiled from ten sediment traps reveal that the seasonal signals preserved in the surface sediments are mostly from spring through autumn. This close relationship between diatom composition and the summer sea surface temperature will be useful in deriving a transfer function in the subarctic North Pacific for the quantitative paleoceanographic and paleoenvironmental studies. The relative abundance of the sea-ice indicator diatoms Fragilariopsis cylindrus and F. oceanica of >20% in the diatom composition is used to represent the winter sea ice edge in the Bering Sea. The northern boundary of the distribution of F. doliolus in the open ocean is suggested to be an indicator of the Subarctic Front, while the abundance of Chaetoceros resting spores may indicate iron input from nearby continents and shelves and induced productivity events in the study area.

LGM, planktic foraminifera (EPILOG version)

[1] We used planktic foraminiferal assemblages in 70 sediment cores from the tropical and subtropical South Atlantic Ocean (10°N-37°S) to estimate annual mean sea surface temperatures (SSTs) and seasonality for the Last Glacial Maximum with a modified version of the Imbrie-Kipp transfer function method (IKTF) that takes into account the abundance of rare but temperature sensitive species. In contrast to CLIMAP Project Members [1981], the reconstructed SSTs indicate cooler glacial SSTs in the entire tropical/subtropical South Atlantic with strongest cooling in the upwelling region off Namibia (7-10°C) and smallest cooling (1-2°C) in the western subtropical gyre. In the western Atlantic, our data support recent temperature estimates from other proxies. In the upwelling regions in the eastern Atlantic, our data conflict with SST reconstructions from alkenones, which may be due to an environmental preference of the alkenone-producing algae or to an underestimation of foraminiferal SSTs due to anomalous high abundances of N. pachyderma (sinistral).

Organic carbon accumulation in the South Atlantic Ocean

A compilation of 1118 surface sediment samples from the South Atlantic was used to map modern seafloor distribution of organic carbon content in this ocean basin. Using new data on Holocene sedimentation rates, we estimated the annual organic carbon accumulation in the pelagic realm (>3000 m water depth) to be approximately 1.8*10**12 g C/year. In the sediments underlying the divergence zone in the Eastern Equatorial Atlantic (EEA), only small amounts of organic carbon accumulate in spite of the high surface water productivity observed in that area. This implies that in the Eastern Equatorial Atlantic, organic carbon accumulation is strongly reduced by efficient degradation of organic matter prior to its burial. During the Last Glacial Maximum (LGM), accumulation of organic carbon was higher than during the mid-Holocene along the continental margins of Africa and South America (Brazil) as well as in the equatorial region. In the Eastern Equatorial Atlantic in particular, large relative differences between LGM and mid-Holocene accumulation rates are found. This is probably to a great extent due to better preservation of organic matter related to changes in bottom water circulation and not just a result of strongly enhanced export productivity during the glacial period. On average, a two- to three-fold increase in organic carbon accumulation during the LGM compared to mid-Holocene conditions can be deduced from our cores. However, for the deep-sea sediments this cannot be solely attributed to a glacial productivity increase, as changes in South Atlantic deep-water circulation seem to result in better organic carbon preservation during the LGM.

Test sizes of planktic foraminifera from the South Atlantic

Geographical size distribution within entire Holocene foraminiferal assemblages is related to global environmental gradients such as temperature, primary productivity, and environmental variability. This study demonstrates that these correlations are also recognizable in late Quaternary assemblages from three locations in the South Atlantic on temporal and latitudinal scales. The size response to temporal paleoenvironmental changes during glacial-interglacial cycles mimics the geographic Holocene size variability. The amplitude of size variability is directly related to the amplitude of the climatic fluctuations as shown by the stable size-temperature relationship over time. The documented changes in the assemblage size are caused by species replacement and intraspecific size variability. The relative importance of these processes depends on the environmental setting. Species have been shown to reach their maximum size and abundance under certain optimum conditions and decrease in size if environmental conditions differ from these optima. We confirm that late Quaternary species sizes were largest at paleotemperatures identical to Holocene ones.

G. bulloides dissolution index of surface sediments

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.

Delta 13C measured on alkenones of surface sediment samples GeoB1008-6 to GeoB3603-1 from the South Atlantic

We have analyzed the stable carbon isotopic composition of the diunsaturated C37 alkenone in 29 surface sediments from the equatorial and South Atlantic Ocean. Our study area covers different oceanographic settings, including sediments from the major upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western South Atlantic. In order to examine the environmental influences on the sedimentary record the alkenone-based carbon isotopic fractionation (Ep) values were correlated with the overlying surface water concentrations of aqueous CO2 ([CO2(aq)]), phosphate, and nitrate. We found Ep positively correlated with 1/[CO2(aq)] and negatively correlated with [PO43-] and [NO3-]. However, the relationship between Ep and 1/[CO2(aq)] is opposite of what is expected from a [CO2(aq)] controlled, diffusive uptake model. Instead, our findings support the theory of Bidigare et al. (1997, doi:10.1029/96GB03939) that the isotopic fractionation in haptophytes is related to nutrient-limited growth rates. The relatively high variability of the Ep-[PO4] relationship in regions with low surface water nutrient concentrations indicates that here other environmental factors also affect the isotopic signal. These factors might be variations in other growth-limiting resources such as light intensity or micronutrient concentrations.