Jan E. Van Hinte

Biogenic carbonate and ice-rafted debris (Heinrich layer) accumulation in deep-sea sediments from a Northeast Atlantic piston core

Abstract Biogenic carbonate mass accumulation rates show distinct Quaternary glacial-interglacial changes in a mid-latitude Northeast Atlantic deep-sea core. During the last 208 kyr, these were generally higher during interglacials averaging 2.2 g/cm 2 kyr than during glacial periods with a maximum average of 1.2 g/cm 2 kyr. Intervals containing high ice-rafted debris have very low biogenic carbonate accumulation rates. Paleo-primary productivity estimates based on biogenic carbonate accumulation rates are generally higher during interglacials averaging 80 gC/m 2 yr than during glacials which average 49 gC/m 2 yr (maximum estimate). Short intervals with possibly reduced primary productivity correspond to Heinrich layers which are the result of pulses of massive iceberg production and rapid melting in the North Atlantic. We identified thirteen Heinrich layers in isotopic stages 7 to 2. They have calendar ages of about 201, 189, 182–183, 164–167, 146–149, 142, 128–131, 64–67, 52–54, 40–43, 29–31, 21 and 15 kyr. These Heinrich layers contain large amounts of ice-rafted debris, including detrital carbonate, low amounts of coccoliths and planktic foraminifera, mainly Neogloboquadrina pachyderma (sinistral). The upper five Heinrich layers are almost barren of coccoliths and the fourth layer is also free of foraminifera. The virtual absence of coccoliths is interpreted to reflect reduced coccolithophorid productivity in the photic zone due to low salinity of surface waters and to blockage of light by ice and suspended materials shed off by melting icebergs. Considering the limited use of other proxies such as organic carbon, diatom and benthic foraminifera due to their very low content/abundance in the sediments, biogenic carbonate mass accumulation rates offer a workable paleo-primary productivity proxy in open-ocean sediments of the carbonate-dominated Northeast Atlantic.

Seasonal stable isotope variations of the modern Amazonian freshwater bivalve Anodontites trapesialis

In a floodplain lake of the Amazon River near the city of Iquitos, northeastern Peru, a one-year monitoring experiment was conducted during which water samples and living bivalves (Anodontites trapesialis) were collected with the aim to investigate seasonal δ18O variation in and fractionation between bivalve aragonite and host water. Both host water and molluscan growth increments show more than 8‰ seasonal variation in δ18O. In the floodplain lake under study the δ18O variation of the water is controlled by contrasting dry and wet season evaporation–precipitation regimes. Molluscan δ18O appears to be in equilibrium with the host water. Although an approximately 4.0‰ offset occurs, δ13C records of water and bivalves are in good agreement, suggesting that both δ18O and δ13C of the shells of freshwater bivalve A. trapesialis are good recorders of (palaeo-)environmental conditions. The δ13C of Dissolved Inorganic Carbon (DIC) is governed by plant growth and/or by changes in aquatic chemistry, affecting the DIC pool.

The fate of coccoliths at 48°N 21°W, Northeastern Atlantic.

Abstract As part of the Joint Global Ocean Flux Study Program (JGOFS) North Atlantic Bloom Experiment (NABE), coccolithophore fluxes were recorded over one-year period, from April 1989 to April 1990. Three vertically moored sediment traps, synchronized on a 14-day interval, were deployed at 48° N 21° W at three water depths, at 1, 2 and 3.7 km (sea floor at 4.4 km). We present the results on the vertical variability of coccolith and coccosphere fluxes through the water column. Calcium carbonate was the largest component of the total mass flux, followed by particulate organic matter and opal. Coccolith and coccosphere seasonal fluxes followed the seasonality of the biogeochemical particle fluxes at all three depths, with maxima occurring during the spring particle “bloom”. The decrease in correlation between coccosphere and coccolith fluxes with depth (from r=0.98 at 1 km depth to 0.60 at 3.7 km depth) documents that disaggregation of the coccospheres occurs during settling in the deeper part of the water column between 1 and 3.7 km water depth. Annual coccosphere and coccolith fluxes were 1.5×108 and 0.9×10 11 m −2 year −1 respectively, at 1 km depth (or 3.1×108 and 1.4×10 11 m −2 year −1 including the spring 1990 episode). For the 3.7 km trap the corresponding values were 0.5×108 and 1.1×1011 (0.6×108 and 1.1×1011 including the partially recorded spring 1990 episode). The slight increase in coccolith numbers with water depth can be explained by coccosphere disintegration and lateral influx. Nevertheless, selective dissolution of delicate coccolith species from 1 to 3.7 km water depth occurs (e.g., relative abundances of Oolithothus fragilis coccoliths decrease from 3% at 1 km to less than 0.5% at 3.7 km water depth). Emiliania huxleyi, Gephyrocapsa muellerae, Calcidiscus leptoporus and Coccolithus pelagicus constitute from 65% (at 1 km depth) to 95% (at 3.7 km depth) of the total coccolith assemblages. The same four coccolith taxa dominate the fossil assemblages in the underlying surface sediments. Selective dissolution continues in the sediments, most notably the abundance of the solution-resistant species C. leptoporus increases from 10 to 15% in the trap samples to 15–40% in the sediments. Annual flux of calcium carbonate in sediment trap samples accounts for about 60% of the total annual mass flux, with the coccolith fraction ( μm ) being about 45% of the annual carbonate flux (mean value of 41% at 1 km and 50% at 3.7 km water depth). The correlation between the μm CaCO 3 and the calculated coccolithophore and calcisphere CaCO3 fluxes decreases with depth (r=0.92 at 1 km and r=0.86 at 3.7 km), indicating that partial dissolution and fragmentation of the delicate, less calcified, coccolithophore and planktic foraminifera species occurs during settling. This fine fragmentation of biogenic CaCO3 can contribute to the μm fraction causing the decrease of correlation, with the calculated CaCO3 fluxes being based only on the flux of intact coccoliths and calcispheres.

A size analysis of planktic foraminifera from the Arabian Sea

Abstract Planktic foraminiferal faunas from different environments in the Arabian Sea were size fractionated using 14 sieves with meshes between 100 and 710 μm, to assess the effect of the sieve mesh size cut off level on the faunal composition and to determine the size frequency distribution of individual species. Nine samples from a plankton pump and a towed net, a sediment trap, a box-core and a piston core were selected, to cover living and settling flux faunas as well as fossil faunas from the sediment. In living faunas, most species show an exponential size frequency distribution, with highest numbers in the finest interval of the size spectrum. In sediment trap and core samples, individual species size frequency distributions may consist of: (1) an exponential distribution of relatively small pre-adult specimens; (2) a Gaussian-shaped distribution of larger specimens, which may be classified as adult or terminal; or (3) a combination of both. The distributions are separated using a best fit technique. The composition of the total planktic foraminiferal fauna strongly changes along the size spectrum. Dominant taxa in >355 μm fractions are Orbulina universa, Globorotalia menardii, Globorotalia tumida, Globigerinella siphonifera and Globigerinoides sacculifer, in 125–355 μm fractions Globigerina bulloides, Globigerinoides ruber, Neogloboquadrina dutertrei and Globigerinita glutinata, and in 100 μm) is present in the 100–125 μm fraction and 1–6% is larger than 250 μm. In samples representing a settling flux (sediment trap and sediment samples) 29–57% of the fauna is present in the 100–125 μm fraction, while 6–23% is larger than 250 μm. Size frequency distributions of the dextral Neogloboquadrina complex (= Neogloboquadrina dutertrei and Neogloboquadrina pachyderma + P–D intergrades) show a bimodal pattern; a smaller peak reflecting dextral Neogloboquadrina pachyderma, and a larger peak of adult Neogloboquadrina dutertrei. By applying a best fit technique to the data, the two species may be separated from each other. In size fractions larger than 150 μm most species have reached the adult stage of ontogeny and we recommend this mesh size for standard faunal analysis. In addition, sieve mesh sizes of 125 and 250 μm have to be used to obtain a reliable estimate of the abundance of small and large species, respectively.

Distributional pattern of planktonic foraminifers and pteropods in surface waters and top core sediments of the Red Sea, and adjacent areas controlled by the monsoonal regime and other ecological factors

Living planktonic foraminiferal and pteropod distribution patterns in the western Arabian Sea, Gulf of Aden and Red Sea, collected during two summer cruises (1984, 1985), reflect the hydrographical system that is mainly controlled by a combination of monsoonal winds and evaporation rates. Spinose species constitute the majority of the planktonic foraminiferal assemblages in the Red Sea during both monsoonal seasons. The non-spinose species Globorotalia menardii, Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, which are always abundant in the Arabian Sea, are present only during winter inflow. The intensity and duration of these inflowing surface currents control their distribution pattern. Stable oxygen isotope ratios show that G. menardii survives but ceases to grow north of Bab el Mandeb, while N. dutertrei continues to grow. Trends in the foraminiferal distribution in surface waters compare well with those of the sea floor, as far as larger specimens (>250 μm) are concerned, but differ for the small ones. Surface distribution patterns of small-sized specimens and juvenile/neanic stages of large-sized fully grown species do not correspond to those in the core top samples. The distribution pattern of living pteropods in the Red Sea is closely related to distinct water masses and corresponds to the distribution in top core sediments. Pteropods are absent in the sediments of the Gulf of Aden and the western Arabian Sea due to dissolution. Peak abundances of various pteropods and foraminifers indicate the presence of local upwelling processes in the Bab el Mandeb area. Determining these dynamics allows for the reconstruction of ancient oceanic environments and climatic interactions in the area.

Centennial-millennial-scale monsoon variations off Somalia over the last 35 ka

Abstract We present a multi-proxy study of sediment Core 905 from the Arabian Sea offshore Somalia to assess the validity of a number of proxies for productivity, temperature and wind strength, to reconstruct the monsoon history in the western Arabian Sea. The present-day seasonal variation in productivity in the modern Arabian Sea off Somalia reflects the change from the high-productivity SW monsoon to the low-productivity NE monsoon seasons. Annual productivity is therefore largely controlled by SW monsoon driven upwelling. The geochemical records of Core 905 document millennial-scale variations, for example, in Ba/Al and Corg content. The Younger Dryas and the time equivalent period to Heinrich event 1 show low annual productivity whereas the early Holocene and Bølling-Allerød periods are characterized by high productivity. The upwelling-productivity peaked during Early Holocene time and was followed by a decrease toward the modern values. The total flux of planktic foraminifera and the concentration of the planktic foraminifera G. bulloides are not always controlled by the total productivity. Variations in calcite dissolution, the advection of expatriate fauna or a seasonal decoupling of primary and secondary production appear to hamper straightforward interpretations of those foraminifera records. We conclude that at significantly changed climatic boundary conditions compared with the present day, bulk-sediment-related proxies of productivity more consistently record the local upwelling history than foraminifer-based productivity proxies.

Notes on the Foraminiferal Genera Laterostomella De Klasz and Rérat and Streptochilus Brönnimann and Resig

the biserial foraminiferal genera Laterostomella de Klasz & Rérat (1962) and Streptochilus Brönnimann & Resig (1971) have been described from the Miocene of Gabon and from Miocene to Recent levels of the Pacific and Indian Oceans, respectively. Both genera have since been found in the Upper Cenozoic of other regions: Laterostomella at Rockall Bank in the North Atlantic and in Papua-New-Guinea, and modern Streptochilus specimens were collected with plankton nets in the northern part of the Indian Ocean. The apertural characteristics of Laterostomella and Streptochilus show marked similarities as does the general shape of Laterostomella guembeliniformis with some Streptochilus species. However, other Laterostomella species have a very different form and isotopic data indicate that Laterostomella has a benthic and Streptochilus a planktic life habitat. We conclude that both genera are valid. For the first time SEM pictures of Laterostomella species are presented to show morphological variability, surface texture and aperture types.

Deep-sea drilling on the upper continental rise off New Jersey, DSDP Sites 604 and 605

Deep Sea Drilling Project Sites 604 and 60S on the upper continental rise are the first of a series of cored holes along the “New Jersey transect” which, when completed, will provide the first comprehensive dipwise suite of drill holes across a passive margin from the coastal plain to the abyssal plain. Our drilling results document the age of important seismic sequence boundaries and allow their correlation with wells on the continental shelf and slope as well as with the regional oceanic seismic stratigraphy. Hole 605,156 km (97 mi) southeast of Atlantic City, New Jersey, and drilled 816.7 m down to mid-Maestrichtian limestones, penetrated a near-complete Cretaceous/Tertiary boundary section overlain by a 200-m expanded Paleocene sequence. Unusually high amounts of terrigenous silts and glauconite are present at the boundary and immediately above. Among the several hypotheses discussed, we suggest that the terrigenous silts and glauconite may represent a high-energy event such as a tsunami caused by a Cretaceous/Tertiary impact. Site 604, 5 km (3 mi) seaward of Site 605, was terminated in upper Miocene glauconitic sands and debris flows at 294.5 m by unstable hole conditions. These sediments contain shelf-derived gravels and exotic blocks of Eocene chalk (up to 50 cm across) eroded from bedrock that is today widely exposed on the adjacent slope. Our drilling results show that denudation of the Eocene units was not limited to the Oligocene A u erosional event, but that major loss occurred during late Miocene and later glacial sea-level lowstands.

DSDP Site 603: First deep (>1000-m) penetration of the continental rise along the passive margin of eastern North America

Drilling at Deep Sea Drilling Project Site 603 has provided the first deep (>1000-m) penetration of strata beneath the continental rise off the Atlantic margin of North America. Nearly continuously cored through 1585 m of section down to Berriasian pelagic limestones, the site 435 km (270 mi) east of Cape Hatteras intersected an extensive Lower Cretaceous deep-sea fan complex, which provides new information on the petroleum potential of the continental rise. Hauterivian to early Aptian in age, this 208-m interval of interbedded limestones, sand, and black shale turbidites begs the existence of any post-Valanginian reefs along the Baltimore Canyon Trough. Less extensive terrigenous turbidites were encountered higher in the section up to the Cretaceous/Tertiary boundary, which is marked by a current-laminated sand rich in dark spherules. Pelagic early Paleogene clays are disconformably overlain by Miocene pelagic mud. Turbiditic silts and clays began to accumulate rapidly at this site during the middle Miocene, leading to deposition of muddy contourites that formed the Lower Continental Rise Hills of the Hatteras Outer Ridge as sand turbidites were ponded concurrently on its landward side. The section at Site 603 confirms the concept that eustatic and other large-scale events subdivide Earth history into distinct chapters allowing the correlation of deep-sea seismic sequence boundaries with continental shelf and margin unconformities.

LATE HOLOCENE ENVIRONMENTAL RECONSTRUCTION OF ST. MICHIEL SALINE LAGOON, CURAÇAO (DUTCH ANTILLES)

Two sediment cores collected from the saline lagoon St. Michiel on Curacao (Dutch Antilles) preserve a approximately 5000-yr record of environmental change. Investigation of radiocarbon-dated sections by accelerator mass spectrometry (AMS) is based on faunal assemblage analyses, sediment mineralogy, and the interpretation of sedimentary facies. The cores recovered from different parts of the lagoon demonstrate different development. Initially, in the proximal part of the lagoon (core STM-2), the sediment accumulated in a coastal, semi-protected bay with strong marine influence, whereas the distal part (STM-1) was dominated by chemical precipitation (gypsum, aragonite). By about 3500-3400 BP, connection with the open sea became very limited due to the gradual formation of a coral rubble barrier at the coastline. Subsequently, the record reveals undisturbed sedimentation in the highly restricted shallow lagoon. Around 1100-1000 BP, biological and sedimentological records indicate a change to less evaporitic conditions. Stages of increased salinity are intercalated with intervals of episodic freshening due to increased runoff and precipitation. The authors demonstrate that since permanent human settlements were established on the island about 1100 BP, the watershed has undergone intensive deforestation, especially during the European colonization at the beginning of the 16th century. Deforestation resulting from agriculture and construction caused increased erosion, which was translated to increased sediment accumulation rates and a shift in lagoon sedimentation from almost entirely endogenic to mostly detrital.