Plankton abundances in water samples of cruise EBENE

The Etude du Broutage en Zone Equatoriale (EBENE) transect (8°S-8°N) explored the equatorial high-nutrient, low-chlorophyll (HNLC) zone and adjacent oligotrophic areas during a La Niña period (October-November 1996). During this time the passage of a tropical instability wave also influenced the region north of the equator. We present a brief summary of EBENE findings, with an emphasis on phytoplankton utilization by the assemblage of protistan and animal consumers. Despite significant variability over the diel cycle, phytoplankton biomass at the equator was relatively constant on a 24-hour timescale, denoting a dynamic balance between growth and losses. The magnitude of the daily cycle in phytoplankton biomass was well constrained by in situ observations of the diel variability in pigments and suspended particulates, by 14C uptake rates from in situ incubations, and from experimental determinations of specific growth and grazing rates. The general equilibrium of production and grazing processes is illustrated by applying biomass-specific grazing rates from the equatorial station to measured planktonic biomass along the EBENE transect and comparing them to measured 14C uptake. Most of the grazing turnover is supported by the production of Prochloroccus (31%) and picoeukaryotic algae (34%). Among the consumers, microzooplankton (<200 µm) account for 59-98% of the grazing losses. The coherence of the results obtained by independent methods suggests that the essential features of the system have been adequately represented by rate and standing stock assessments from the EBENE study.

Biomass of mesoplankton and macroplankton in the central tropical and equatorial regions of the Pacific Ocean

Collections made with 150 l sampling bottles and BR 113/140 nets, as well as direct counts from the Mir submersible are used to analyze vertical distribution of total biomass of meso- and macroplankton and biomass distributions of their main component groups in the central oligotrophic regions of the North Pacific. Biomass of mesoplankton in the upper 200 m layer ranges from 3.1 to 8.6 g/m**2, but sometimes it increases up to as much as 98 g/m**2 in local population explosions of salps. Jellies predominate in macroplankton at depths of up to 2-3 km, contributing 97-98% of live weight and 30-70% of biomass as organic carbon. In importance they are followed by micronecton fishes (up to 40% of organic carbon). Contributions of other groups countable from the submersible were negligible. Distributions of species at particular stations are discussed.

Late Cretaceous benthic foraminiferal assemblages from Demerara Rise, western tropical Atlantic

This paper is based on Santonian-Campanian sediments of Ocean Drilling Program Sites 1257 (2951 mbsl) and 1259 (2353 mbsl) from Demerara Rise (Leg 207, western tropical Atlantic, off Surinam). According to its position, Demerara Rise should have been influenced by the early opening of the Equatorial Atlantic Gateway and the establishment of a bottom-water connection between the central and South Atlantic Oceans during the Late Cretaceous. The investigated benthic foraminiferal faunas demonstrate strong fluctuations in bottom-water oxygenation and organic-matter flux to the sea-floor. The Santonian-earliest Campanian interval is characterised by laminated black shales without benthic foraminifera in the lowermost part, followed by an increasing number of benthic foraminifera. These are indicative of anoxic to dysoxic bottom waters, high organic-matter fluxes and a position within the oxygen minimum zone. At the shallower Site 1259, benthic foraminifera occurred earlier (Santonian) than at the deeper Site 1257 (Early Campanian). This suggests that the shallower site was characterised by fluctuations in the oxygen minimum zone and that a re-oxygenation of the sea-floor started considerably earlier at shallower water-depths. We speculate that this re-oxygenation was related to the ongoing opening of the Equatorial Atlantic Gateway. A condensed glauconitic chalk interval of Early Campanian age (Nannofossil Zone CC18 of Sissingh) overlies the laminated shales at both sites. This interval contains benthic foraminiferal faunas reflecting increasing bottom-water oxygenation and reduced organic-matter flux. This glauconitic chalk is strongly condensed and contains most of the Lower and mid-Campanian. Benthic foraminiferal species indicative of well-oxygenated and more oligotrophic environments characterise the overlying mid- to Upper Campanian nannofossil chalk. During deposition of the nannofossil chalk, a permanent deep-water connection between the central and South Atlantic Oceans is proposed, leading to ventilated and well-oxygenated bottom waters. If this speculation is true, the establishment of a permanent deep-water connection between the central and South Atlantic Oceans terminated Oceanic Anoxic Event 3 "black shale" formation in the central and South Atlantic marginal basins during the Early Campanian (Nannofossil Zone CC18) and led to well-oxygenated bottom waters in the entire Atlantic Ocean during the Late Campanian (at least from Nannofossil Zone CC22 onwards).

Calcareous nannofossil during the Paleocene Eocene Thermal Maximum in sediments of Walvish Ridge

Compositions and abundances of calcareous nannofossil taxa have been determined in a ca 170 kyrs long time interval across the Paleocene/Eocene boundary at 1-cm to 10-cm resolution from two ODP Sites (1262, 1263) drilled along the flank of the Walvis Ridge in the South Atlantic. The results are compared to published data from ODP Site 690 in the Weddell Sea. The assemblages underwent rapid evolution over a 74 kyrs period, indicating stressed, unstable and/or extreme photic zone environments during the PETM hyperthermal. This rapid evolution, which created 5 distinct stratigraphic horizons, is consistent with the restricted brief occurrences of malformed and/or weakly calcified morphotypes. The production of these aberrant morphotypes is possibly caused by major global scale changes in carbon cycling in the ocean-atmosphere system, affecting also photic zone environments. No marked paleoecologically induced changes are observed in abundances of the genera Discoaster, Fasciculithus and Sphenolithus at the Walvis Ridge sites. Surprisingly, there is no significant correlation in abundance between these three genera, presumed to have had a similar paleoecological preference for warm and oligotrophic conditions.

Benthic foraminifera across the Cretaceous/Paleogene boundary in ODP Hole 198-1210A

Deep-sea benthic foraminifera show important but transient assemblage changes at the Cretaceous/Paleogene (K/Pg) boundary, when many biota suffered severe extinction. We quantitatively analyzed benthic foraminiferal assemblages from lower bathyal-upper abyssal (1500-2000 m) northwest Pacific ODP Site 1210 (Shatsky Rise) and compared the results with published data on assemblages at lower bathyal (~ 1500 m) Pacific DSDP Site 465 (Hess Rise) to gain insight in paleoecological and paleoenvironmental changes at that time. At both sites, diversity and heterogeneity rapidly decreased across the K/Pg boundary, then recovered. Species assemblages at both sites show a similar pattern of turnover from the uppermost Maastrichtian into the lowermost Danian: 1) The relative abundance of buliminids (indicative of a generally high food supply) increases towards the uppermost Cretaceous, and peaks rapidly just above the K/Pg boundary, coeval with a peak in benthic foraminiferal accumulation rate (BFAR), a proxy for food supply. 2) A peak in relative abundance of Stensioeina beccariiformis, a cosmopolitan form generally more common at the middle than at the lower bathyal sites, occurs just above the buliminid peak. 3) The relative abundance of Nuttallides truempyi, a more oligotrophic form, decreases at the boundary, then increases above the peak in Stensioeina beccariiformis. The food supply to the deep sea in the Pacific Ocean thus apparently increased rather than decreased in the earliest Danian. The low benthic diversity during a time of high food supply indicates a stressed environment. This stress might have been caused by reorganization of the planktic ecosystem: primary producer niches vacated by the mass extinction of calcifying nannoplankton may have been rapidly (<10 kyr) filled by other, possibly opportunistic, primary producers, leading to delivery of another type of food, and/or irregular food delivery through a succession of opportunistic blooms. The deep-sea benthic foraminiferal data thus are in strong disagreement with the widely accepted hypothesis that the global deep-sea floor became severely food-depleted following the K/Pg extinction due to the mass extinction of primary producers ("Strangelove Ocean Model") or to the collapse of the biotic pump ("Living Ocean Model").

Palynology and dinoflagellate counts of surface sediments, and sea surface conditions, of sites in the North Pacific

Palynological analyses were performed on 53 surface sediment samples from the North Pacific Ocean, including the Bering and Okhotsk Seas (37-64°N, 144°E-148°W), in order to document the relationships between the dinocyst distribution and sea-surface conditions (temperatures, salinities, primary productivity and sea-ice cover). Samples are characterized by concentrations ranging from 18 to 143816 cysts/cm**3 and the occurrence of 32 species. A canonical correspondence analysis (CCA) was carried out to determine the relationship between environmental variables and the distribution of dinocyst taxa. The first and second axes represent, respectively, 47% and 17.8% of the canonical variance. Axis 1 is positively correlated with all parameters except to the sea-ice and primary productivity in August, which are on the negative side. Results indicate that the composition of dinocyst assemblages is mostly controlled by temperature and that all environmental variables are correlated together. The CCA distinguishes 3 groups of dinocysts: the heterotrophic taxa, the genera Impagidinium and Spiniferites as well as the cyst of Pentapharsodinium dalei and Operculodinium centrocarpum. Five assemblage zones can be distinguished: 1) the Okhotsk Sea zone, which is associated to temperate and eutrophic conditions, seasonal upwellings and Amur River discharges. It is characterized by the dominance of O. centrocarpum, Brigantedinium spp. and Islandinium minutum; 2) the Western Subarctic Gyre zone with subpolar and mesotrophic conditions due to the Kamchatka Current and Alaska Stream inflows. Assemblages are dominated by Nematosphaeropsis labyrinthus, Pyxidinopsis reticulata and Brigantedinium spp.; 3) the Bering Sea zone, depicting a subpolar environment, influenced by seasonal upwellings and inputs from the Anadyr and Yukon Rivers. It is characterized by the dominance of I. minutum and Brigantedinium spp.; 4) the Alaska Gyre zone with temperate conditions and nutrient-enriched surface waters, which is dominated by N. labyrinthus and Brigantedinium spp. and 5) the Kuroshio Extension-North Pacific-Subarctic Current zone characterized by a subtropical and oligotrophic environment, which is dominated by O. centrocarpum, N. labyrinthus and warm taxa of the genus Impagidinium. Transfer functions were tested using the modern analog technique (MAT) on the North Pacific Ocean (= 359 sites) and the entire Northern Hemisphere databases ( = 1419 sites). Results confirm that the updated Northern Hemisphere database is suitable for further paleoenvironmental reconstructions, and the best results are obtained for temperatures with an accuracy of +/-1.7 °C.

Pollen and diatom analysis on varved sediments in Lake Jues (Harz Mountains, Germany)

Studies combining sedimentological and biological evidence to reconstruct Holocene climate beyond the major changes, and especially seasonality, are rare in Europe, and are nearly completely absent in Germany. The present study tries to reconstruct changes of seasonality from evidence of annual algal successions within the framework of well-established pollen zonation and 14C-AMS dates from terrestrial plants. Laminated Holocene sediments in Lake Jues (10°20.70' E, 51°39.30' N, 241 m a.s.l.), located at the SW margin of the Harz Mountains, central Germany, were studied for sediment characteristics, pollen, diatoms and coccal green algae. An age model is based on 21 calibrated AMS radiocarbon dates from terrestrial plants. The sedimentary record covers the entire Holocene period. Trophic status and circulation/stagnation patterns of the lake were inferred from algal assemblages, the subannual structure of varves and the physico-chemical properties of the sediment. During the Holocene, mixing conditions alternated between di-, oligo- and meromictic depending on length and variability of spring and fall periods, and the stability of winter and summer weather. The trophic state was controlled by nutrient input, circulation patterns and the temperature-dependent rates of organic production and mineralization. Climate shifts, mainly in phase with those recorded from other European regions, are inferred from changing limnological conditions and terrestrial vegetation. Significant changes occurred at 11,600 cal. yr. BP (Preboreal warming), between 10,600 and 10,100 cal. yr. BP (Boreal cooling), and between 8,400 and 4,550 cal. yr. BP (warm and dry interval of the Atlantic). Since 4,550 cal. yr. BP the climate became gradually cooler, wetter and more oceanic. This trend was interrupted by warmer and dryer phases between 3,440 and 2,850 cal. yr. BP and, likely, between 2,500 and 2,250 cal. yr. BP.

Geochemistry of sediment cores from METEOR cruise M76/1b

Marine sediments are the main sink in the oceanic phosphorus (P) cycle. The activity of benthic microorganisms is decisive for regeneration, reflux, or burial of inorganic phosphate (Pi), which has a strong impact on marine productivity. Recent formation of phosphorites on the continental shelf and a succession of different sedimentary environments make the Benguela upwelling system a prime region for studying the role of microbes in P biogeochemistry. The oxygen isotope signature of pore water phosphate (d18OP) carries characteristic information of microbial P cycling: Intracellular turnover of phosphorylated biomolecules results in isotopic equilibrium with ambient water, while enzymatic regeneration of Pi from organic matter produces distinct offsets from equilibrium. The balance of these two processes is the major control for d18OP. Our study assesses the importance of microbial P cycling relative to regeneration of Pi from organic matter from a transect across the Namibian continental shelf and slope by combining pore water chemistry (sulfate, sulfide, ferrous iron, Pi), steady-state turnover rate modeling, and oxygen isotope geochemistry of Pi. We found d18OP values in a range from 12.8 per mill to 26.6 per mill, both in equilibrium as well as pronounced disequilibrium with water. Our data show a trend towards regeneration signatures (disequilibrium) under low mineralization activity and low Pi concentrations, and microbial turnover signatures (equilibrium) under high mineralization activity and high Pi concentrations. These findings are opposite to observations from water column studies where regeneration signatures were found to coincide with high mineralization activity and high Pi concentrations. It appears that preferential Pi regeneration in marine sediments does not necessarily coincide with a disequilibrium d18OP signature. We propose that microbial Pi uptake strategies, which are controlled by Pi availability, are decisive for the alteration of the isotope signature. This hypothesis is supported by the observation of efficient microbial Pi turnover (equilibrium signatures) in the phosphogenic sediments of the Benguela upwelling system.