Box Core Samples Research Articles

Luminescence-dating zeroing tests in Lake Hoare, Taylor Valley, Antarctica

In two of the perennially ice-covered lakes in the McMurdo Dry Valleys, lakes Hoare and Bonney in the Taylor Valley, bottom water has 14C ages of ∼2.7 ka and ∼10 ka (respectively), rendering 14C ages of bottom sediments highly problematic. Consequently, we tested the effectiveness of thermoluminescence (TL) zeroing in polymineral fine silt material from several depositional environments around and on the lake (stream suspensions, ice-surface sand dune, and silty sand from near the top of the more-than-3m-thick ice). We also conducted TL and infrared-stimulated-luminescence (IRSL) dating tests on material from three box cores recovered from the bottom of Lake Hoare, in a transect away from the abutting Canada Glacier. We observed effective zeroing of light-sensitive TL in suspended silt from one input stream and less effective zeroing from another stream. We observed effective zeroing of light-sensitive TL also in silt from a glacier-proximal eolian ice-surface dune and from sand from within the upper 5 cm of ice. In contrast, in box-core 1, the bottom sediment yielded minimum TL apparent ages of 1500-2600 yrs, with no discernable stratigraphic depth trend. IRSL dating applied to the same box-core samples produced significantly lower age estimates, ranging from ~600 ± 200 yrs to 1440 ± 270 yrs top-to-bottom, an improvement over the depth-constant ~2200 yrs TL ages. In two other cores closer to the Canada Glacier, IRSL ages from ~600 ± 200 yrs (top) to ~ 2900± 300 yrs (at depth) were measured. Not only are the IRSL ages a significant improvement over the TL results, but the near-core-top IRSL ages are also a dramatic improvement over the 14C results (~2.7 ka). IRSL dating has a demonstrated potential to supplant 14C dating for such antarctic lacustrine deposits.

A Nondestructive Technique for Predicting the In Situ Void Ratio for Marine Sediments

This study tests the hypothesis that the in situ void ratio of surficial marine sediments may be predicted from shear wave velocity-depth data with a reliability equal to that of other methods currently available. Shear wave velocity is fundamentally controlled by the number of grain-to-grain contacts per unit volume of material and by the effective stress across those contacts. In this study, three previously established empirical formulae are used to predict void ratio from velocity-depth data. Field data were acquired along a transect off the northern Californian coast across which water depth increased from 35 to 70 m and seafloor sediment type varied from sand to silty-sand, respectively. A towed seafloor sled device was used to collect shear wave refraction data, and a marked, systematic decrease in velocity was observed along the line, ranging from 35–70 m/s for the coarse, near-shore material to 25–40 m/s for the finer, offshore deposits. Void ratios predicted from these velocities were compared with data measured directly from box-core samples. Of the formulae used for prediction, two agree remarkably well with the control data. Both predicted and control values increase from 0.6–0.8 for the sandy material to 1.1–1.5 for the silty-sand. Thus, this study does not disprove the hypothesis set and demonstrates the potential of field shear wave velocity-depth data as a means of delineating spatial variation in void ratio for surficial marine sediments in a remote, nondestructive manner.

Patterns in deep-sea macrobenthos at the continental margin: standing crop, diversity and faunal change on the continental slope off Scotland

Depth-related patterns of macrobenthic community structure and composition have been studied from box-core samples from the Scottish continental slope where deep-sea trawling and oil exploration are becoming increasingly important. There is a strong pattern of declining biomass and faunal abundance with increasing depth, but results also indicate reduced biomass and numbers of macrobenthos in the shallowest samples from just below the shelf edge where there are coarse sediments and a regime of strong bottom currents. There is also reduced species diversity at the shallowest stations, probably caused by hydrodynamic disturbance, but no clear mid-slope peak in species diversity as described from the northwest Atlantic. Taxonomic composition of the macrobenthic community shows most change between about 1000 and 1200 m, expressed as a major dichotomy in multivariate analysis by cluster analysis and ordination. It also shows up as a step-like increase in the rate of accumulation of new macrofaunal species. This corresponds to a change in hydrodynamic regime, from a seabed rich in suspension- and interface-feeding epifauna, to one where biogenic traces from large, burrowing deposit feeders are well developed, and visible epifauna rare in seabed photographs. It also corresponds to the depth zone where earlier study of megafaunal echinoderms in trawl and epibenthic sled samples also shows a clear peak in across-slope rate of change in faunal composition.

Benthic community productivity in the Magellan Region and in the Weddell Sea

Our comparison of macrobenthic biomass, production and productivity of the Magellan region (14 - 349 m water depth) and the Weddell Sea (132 - 548 m water depth) is based on multi box corer samples collected in both areas. Biomass is slightly but not significantly lower in the Magellan region (7.3 g C m-2) than in the Weddell Sea (12.0 g C m-2). Annual production and P/B ratio are higher in the Magellan region (5.1 g C m-2 y-1, 0.7 y-1) as compared to the Weddell Sea (3.6 g C m-2 y-1, 0.3 y-1). In the Magellan region, Mollusca, Polychaeta and Arthropoda dominate benthic production, whereas in the Weddell Sea Polychaeta, Porifera and Echinodermata are the most productive taxa.

A large testate protist, Gromia sphaerica sp. nov. (Order Filosea), from the bathyal Arabian Sea

The deep ocean provides a habitat for several large testate protistan taxa, e.g., xenophyophores and komokiaceans. However, testate amoebae (orders Lobosea and Filosea) have never been reliably reported from below 270 m depth. Here we describe large spherical protists, up to almost 40 mm diameter, recovered in box core, multicore, and epibenthic sledge samples taken on the continental slope off Oman (NW Arabian Sea) during RRS Discovery Cruise 211. Specimens consist of an organic envelope, with numerous tiny apertures, enclosing a mass of fine waste pellets (stercomata). On the basis of its wall ultrastructure (revealed by high-voltage transmission electron microscopy), this organism is identified as a member of the filosean genus Gromia. It is described herein as Gromia sphaerica sp. nov. The wall is layered and includes the `honeycomb membrane', a structure that is unique to this genus. The new species is the first gromiid to be reported from the deep sea. It lives between about 1200 and 1630 m below the main oxygen minimum zone, in an area where bottom-water oxygen concentrations are around 0.47 ml l −1. The new gromiid species is clearly visible in bottom photographs taken at 1273 m (2.7 individuals m −2), 1318 m (2.7 indiv. m −2), 1624 m (17.3 indiv. m −2) and 1633 m (76.5 indiv. m −2). They lie partly embedded in the seafloor and are sometimes grouped in twos and threes. Specimens are typically surrounded by an apron of lighter-coloured sediment, possibly resulting from their feeding activities. Where they lie in very close proximity, the apron around one specimen typically obscures adjacent tests.

Micro-scale geotechnical variability in continental slope and abyssal sediments influenced by the oxygen minimum zone in the Arabian Sea

We describe a new micro-scale method of measuring the penetration resistance of the surficial layers of slope and abyssal plain sediments within and below the OMZ on the Oman continental slope. The micro-scale load resistance penetrometer (MLRP) continuously measures the penetration resistance ( q) of a cylindrical probe driven at constant velocity into cores of sediment recovered by sub-coring box core samples. Measurements are recorded at 0.8 mm depth intervals into the sediment by an automated data acquisition system. These provide continuous depth profiles of penetration resistance ( q) and its variability ( qr) in surficial sediments (0–100 mm). The results represent the first published application of this technique to slope and abyssal plain sediments. The q data divide the stations into shallow OMZ (SOMZ) stations (391–854 m), deep OMZ (DOMZ) stations (987–1285 m), and abyssal non-OMZ (ANOMZ) stations (3392–3396 m). One station (688 m) was anomalous. Penetration resistance ( q) was positively related to shear strength and inversely related to water content, but not to bioturbation measured as burrow numbers. Relaxation and pull-out data were obtained from three stations. Three patterns of qr were observed, consisting of well-confined data, data containing large negative and positive spikes, and data showing a progressive down-core increase in spikes. Four additional measures of q variability were defined, which show statistically significant relationships with burrow numbers. An increase in bioturbation, estimated by burrow numbers, is associated with a decrease in micro-scale variability in q. This is related to the activity of burrowing macrofauna. Our results are discussed in the context of biogeotechnical modifications of sedimentary environments.

Variations in bioturbation across the oxygen minimum zone in the northwest Arabian Sea

Oxygen minimum zones are expected to alter substantially the nature, rates and depths of bioturbation along continental margins, yet these effects remain poorly studied. Using excess 210 Pb profiles, sediment X-radiography and box-core samples for macrofauna, we examined bioturbation processes at six stations (400, 700, 850, 1000, 1250 and 3400 m deep) along a transect across the oxygen minimum zone (OMZ) on the Oman margin. Bottom-water oxygen concentrations ranged from ∼0.13 ml l −1 at 400 m to ∼2.99 ml l −1 at 3400 m. 210 Pb mixed-layer depth and bioturbation intensity ( D b) exhibited high within-station variance, and means did not differ significantly among stations. However, the mean mixed-layer depth (4.6 cm) for pooled OMZ stations (400–1000 m depths, 0.13–0.27 ml l −1 bottom-water oxygen) was half that for stations from similar water depths along well-oxygenated Atlantic and Pacific slopes (11.1 cm), suggesting that oxygen stress reduced 210 Pb mixing depth on the Oman margin. Modal burrow diameter and the diversity of burrow types at a station were highly correlated with bottom-water oxygen concentration from the edge to the core of the Oman OMZ (Spearman's rho⩾0.89, p⩽0.02), suggesting that these parameters are useful proxies for bottom-water oxygen concentrations under dysaerobic conditions. In contrast, neither the maximum diameter and nor the maximum penetration depth of open burrows exhibited oxygen-related patterns along the transect. Reduced 210 Pb mixing depth within the Oman-margin OMZ appeared to result from a predominance of surface-deposit feeders and tube builders within this zone, rather than from simple changes in horizontal or vertical distributions of macrofaunal abundance or biomass. The number of burrow types per station was highly correlated with macrofaunal species diversity, suggesting that burrow diversity may be a good proxy for species diversity in paleo-dysaerobic assemblages. We conclude that bottom-water oxygen concentrations of 0.13–0.27 ml l −1 substantially alter a number of bioturbation parameters of importance to diagenetic and biofacies models for continental margins.

Sedimentary fluxes of 90Sr, 137Cs, 239,240Pu and 210Pb in the East Sea (Sea of Japan)

Sediment cores collected from the deep basins of the East Sea (Sea of Japan) provide an ongoing and historical record of artificial radionuclides contamination into one of the most highly publicized radioactive waste dumping areas in the world ocean. The depth distributions of 90Sr, 137Cs and 239,240Pu in sediment cores were investigated with the aid of 210Pb-derived sediment accumulation and mixing rates in the deep basins of the East Sea (Sea of Japan). Five box core samples were collected from the northern Yamato Ridge, Korea Plateau, Ulleung and Japan Basins below 1000-m depth. Sediment inventories of 137Cs and 239,240Pu are inversely correlated with water depth and linearly correlated with sediment accumulation rates. The inventories of these nuclides are linearly correlated with the accumulation rates of organic carbon in sediments. The 238Pu/ 239,240Pu activity ratios in sediments are 0.036±0.009 suggesting that most of the Pu to the study area is derived from the global fallout. The activity ratios of 239,240Pu/ 137Cs, and 90Sr/ 137Cs in bottom sediments are much lower than those of global fallout due to the differences of particle affinity and biological uptake of these nuclides. Sediment inventories of 90Sr and 137Cs constitute <4% of the anticipated inventories from the global fallout, while those of 239,240Pu constitute 30–150% of the anticipated inventories from the global fallout. The residence time of the dissolved 239,240Pu in the study area is estimated to be 200–400 years based on the sediment inventory and/or sediment accumulation rate, and water column inventory.

Macrofaunal abundance at 79°N off East Greenland: opposing data from epibenthic-sledge and box-corer samples

During the expedition ARK XI-2 with RV Polarstern in September/October 1995, a transect of samples was taken off East Greenland from the shelf down to about 1800 m depth by means of an epibenthic sledge and a giant box corer, in order to obtain information on epibenthic and endobenthic macrofauna. Within the epibenthic sledge catches, the Cumacea were the most numerous taxon with 32,123 (always calculated for 1000 m) specimens, followed by the Isopoda with 26,914 specimens, and Amphipoda with 20,900; Mysidacea and Tanaidacea were less important. Amphipoda were the most numerous taxon on the shelf, while with increasing depth Isopoda and especially Cumacea became increasingly important. Macrofauna analysed from the box-corer samples decreased in abundance with depth. The highest macrofaunal abundance was found on the shelf, with 8128 ind. m−2 (200 m); below this depth, density decreased from ≈2000 ind. m−2 (at 800 m) to ≈800 ind. m−2 (2000 m). Peracarid crustaceans and polychaetes were, next to bivalves, the most frequent. Polychaetes clearly dominated the shelf communities; their relative percentage decreased with increasing water depth while the relative percentage of Peracarida increased simultaneously. Interestingly, the highest number of peracarids in the epibenthic-sledge samples was reported from deep sea, while box-corer samples showed decreasing macrofaunal abundance with depth. The relative percentage of peracarids in the box-corer samples increased with depth, while in absolute numbers, peracarids from these samples were also highest on the shelf at about 200 m depth.

Dispersal and deposition of suspended sediment on the shelf off the Tagus and Sado estuaries, S.W. Portugal

The Portuguese margin in front of the Tagus and Sado rivers is characterized by a narrow shelf incised by numerous canyons and by a large mud deposit. The two estuaries that feed this continental margin have distinct impact. The suspended particulate matter concentration values in the mouth of the Tagus are four times higher than in the Sado. During the summer the surface nepheloid layer is always larger than during the winter when it is restricted near the mouth of the estuary. This nepheloid layer may reach 30 km in length extending westward. The bottom nepheloid layer usually shows higher nephelometer values, and has a typical distribution: it is usually diverted southward in the direction of the Lisbon Submarine Canyon. We estimate the amount of suspended matter being discharged annually from the Tagus estuary to be between 0.4 and 1×10 6 t. The area covered by fine deposits is about 560 km 2. Hence the thickness of sediments deposited annually should be between 0.07 and 0.18 cm. The sedimentation rates calculated from the 210Pb excess vary between 0.16 and 2.13 cm y −1 which correspond to the maximum rate. For a layer of 1 cm thick, 81,000 t of particulate organic carbon (POC) should be trapped. That would represent, with a minimum sedimentation rate between 0.07 and 0.18 cm y −1, an entrapment of 6000–15,000 t POC y −1. The trace metals content of box core samples clearly shows the anthropogenic impact in the uppermost level (5 cm thick) in the Tagus estuary and in all the sedimentary deposits (15 cm thick) on the shelf muddy area. Despite the narrowness of the shelf, a significant part of continental fluxes fails to reach the deep ocean.

Spatial heterogeneity of acoustic bottom backscattering model parameters and predicted results

Sediment geoacoustic and roughness measurements were made during a mine burial experiment in the North Sea in November 1997. The experiment site was a sand ridge (20.5–32-m water depth) influenced by strong tidal currents and wave action. Stereo photographs indicated that the sediment roughness was characterized by cross-rippled fine sand that transitioned to straight- and sinuous-rippled mixtures of fine quartz sand and mollusk shell fragments. Box core samples were subsampled with core tubes from which measurements of sediment compressional wave velocity and attenuation, density, and grain size distribution were made. Values for sediment velocity ratio, density ratio, slope of the roughness power spectrum, roughness spectral strength, and other measured and calculated parameters were used as input parameters for the composite roughness model to predict bottom backscattering strengths from various regions within the experiment site. Horizontal and vertical variability in sediment properties is high relative to other shallow sites under the influence of strong hydrodynamic forcing. There is a marked interdependence among sediment geoacoustic properties, water depth and bottom roughness features. The influence of hydrodynamic conditions and sediment type on the acoustic model predictions is demonstrated by the spatial variability in predicted scattering of high-frequency sound from the site.

Consolidation and Permeability Behavior of High Porosity Baltic Seabed Sediments

Abstract The consolidation and permeability (hydraulic conductivity) characteristics of the high-porosity surficial seabed sediments from a test site in the Eckernförde Bay, Baltic Sea, were studied using a constant rate of strain (CRS) consolidation and hydraulic conductivity testing system employing flow pumps. These silty clay sediments are characterized by high void ratios, high organic content, and variable concentration of methane gas. Results from CRS consolidation tests show appreciable apparent overconsolidation of the sediments at the upper 200 cm with a gradual transition to normally consolidated states below 250 cm in depth. The highly compressible nature of the material is reflected in the large values of the compression index. The hydraulic conductivity of the box core samples ranges from 2.5 × 10−7 to 3.0 × 10−6 cm/s for void ratios ranging from 4.5 to 7.0 with a linear relationship between void ratio and log of hydraulic conductivity. The engineering behavior of these sediments is of practical significance in geoacoustic modeling of the seafloor for studies related to mine countermeasure programs.

The depth distribution of Ostracoda from the Greenland Sea

Abstract. Sixteen box core samples of Recent sediment collected in three transects across the East Greenland shelf and slope were analysed for Ostracoda. The samples which range in depth from 274 m to 3355 m, yielded a total of 52 species belonging to 25 genera. No less than 26 of the species also occur in the adjacent Scoresby Sund fjord complex. The fauna represents an interesting mixture of high latitude shallow water Arctic species and others known from bathyal and abyssal depths in the North Atlantic, including some pandemic deep sea species. Many species occur in shallower water here than in the North Atlantic due to the colder water. The fauna comprised of three associations (Shelf/Upper Slope; Slope: Lower Slope/Abyss). A marked faunal turnover occurs at the Upper Slope. The study raises questions concerning the constancy and universality of the relationship between certain ostracod species and water masses.

Deep-sea benthic foraminiferal species diversity in the NE Atlantic and NW Arabian sea: a synthesis

We present a synthesis of species diversity data (Fishers’ alpha index, Shannon–Wiener (log 2), ES(100), Rank 1 Dominance) for “live” (stained) foraminifera from five bathyal (1340 m depth) and abyssal (4450–4950 m depth) sites in the NE Atlantic and a 3400 m-deep site in the Arabian Sea. Three Atlantic sites (Porcupine Seabight, BIOTRANS, Porcupine Abyssal Plain) are subject to seasonal phytodetritus inputs that support low diversity populations (8–17 species). In other respects the foraminifera are highly diverse. The meiofaunal fractions (>45 or >63 μm; including fragmented and phytodetritus species) of abyssal Atlantic samples yielded >110 and >170 species in the 0–1 cm and 0–10 cm layers, respectively; the Arabian Sea sample (0–1 cm layer only) yielded 232 species. In both cases, values for diversity measures were very high. Diversity was rather lower in bathyal Porcupine Seabight samples (0–1 cm layer), which yielded <100 species. The foraminiferal macrofauna (>500 μm; Porcupine and Madeira Abyssal Plains) was also speciose (113–133 species), but diversity measures were lower and dominance higher than for the meiofauna. All assemblages contained numerous undescribed species, many belonging to poorly known monothalamous, soft-bodied taxa. Sample diversity was influenced by several factors. Combining phytodetrital and sediment populations reduced diversity and increased dominance slightly; the inclusion of deeper sediment layers and finer sieve fractions had the opposite effect. The inclusion of fragments had more impact on macrofaunal than on meiofaunal diversity, although in both cases the effect was inconsistent (either positive or negative). Porcupine and Madeira Abyssal Plain multicore samples (>63 μm fraction) contained substantially more foraminiferal species than nematode species; the numbers of foraminiferal species in boxcore samples (>500 μm fraction) were comparable to, or greater than, literature values for macrofaunal taxa such as polychaetes and isopods. Few of the more abundant species at our Atlantic abyssal plain and Arabian Sea sites are endemic. This is consistent with literature evidence that many common deep-sea foraminiferal species are cosmopolitan and implies that global foraminiferal diversity may be more modest than the high sample diversity might suggest. Calcareous foraminifera, which are well-known taxonomically and have a good fossil record, may provide a model for diversity patterns among the deep-sea benthic biota in general.

Macroinfauna of a tropical fjord-like embayment: Golfo Dulce, Costa Rica

A total of 136 sediment subsamples was analyzed (500 μm mesh screen) for benthic invertebrates from 26 box corer samples collected by the RV Victor Hensen (Dec. 7, 8, 9, 1993), at nine stations ranging in depth from 43 to 200 m in the Golfo Dulce embayment. Pacific coast of Costa Rica. A total of 1690 individuals and 69 species was found; polychaetes dominated the fauna in terms of number of individuals (1506) and species (47). Eight species of polychaetes accounted for 73% of the total number of individuals: Prionospio (Minuspio) sp. A (19.53%) Aricidea (Acesta) catherinae (13.31%), Levinsenia gracilis (11.00%), Aphelochaeta longisetosa (10.18%), Paraprionospio pinnata (6.63%) Cossura brunnea (5.27%), Mediomastus californiensis (3.61%), and Scoletoma platylobata (3.43%). No organisms were collected at the two deepest statiQns (200 m) located inside Golfo Dulce; however, a diverse fauna was found at similar depths II the embayment mouth and in waters shallower than 100 m inside Golfo Dulce. The station located at the sill (75 m) was the most diverse (37 species). The species composition of the fauna found during this study has little resemblance to that reported for Golfo Dulce in 1976. Diversity (H') and Evenness (J'), however, were similar in both surveys. The results of cluster analyses performed on the 1976 and 1993 data sets yielded two main groups of stations. One group was made of the stations located at the mouth and shelf, while another group included those located inside Golfo Dulce. The fauna of the sill sediments may represent a transition zone between the environments characteristic of the entrance to Golfo Dulce and those inside the embayment. The fluctuating bottom conditions of hypoxia-anoxia characteristic of most of Golfo Dulce, in addition to the likely impact of strong El Nino in the marine ecosystems, might be the causes for drastic shifts in species composition and abundance. Golfo Dulce is still a relatively unpolluted embayment, but an Integrated Area Management Plan is urgently needed. This paper, in addition to those already published resulting from the RV Victor Hensen Expedition, are a step towards this goal.

Sarsicopia polaris gen. et sp.n., the first platycopioida (Copepoda: Crustacea) from the Arctic Ocean, and its phylogenetic significance

A new genus of Platycopioida is described from a boxcore sample taken at a depth of 534 m in the ArcticBarents Sea. This is the deepest record ofPlatycopioida so far. Sarsicopia gen. n. is thesistergroup of a taxon comprising Platycopia and Nanocopia; the sistergroup ofthese is Antrisocopia. Sarsicopia gen. n.is the only platycopioid to retain 2 inner setae onthe second endopod segment P2–P4, and 8 setae in thethird endopod segment of P2. The male antennnule isremarkable in having a geniculation located betweenancestral segments XX and XXI. It is suggested thatthis flexure zone was already present in thegroundpattern of Copepoda. Platycopia and Nanocopia have secondarily lost thisgeniculation.

Basing conservation policies for the deep-sea floor on current-diversity concepts: a consideration of rarity

Exploitation of deep-sea resources is now underway and there is economic pressure to renew and expand currently restricted waste disposal in that environment. Since the deep sea is noted for very high species diversity, it is appropriate that diversity conservation be initiated. Review of current concepts of diversity maintenance finds that the ideas have evolved more through increasing information about sources of heterogeneity than through rigorous testing. This history weakens the immediate value of these concepts for the development of conservation strategies and demonstrates the need for additional investigation. Such inquiry might focus upon the rare component of overall species richness. A comparison of box core samples at 2100m in the western Atlantic and the Gulf of Mexico continental shelf reveals that deep soft bottoms are not unique in having many rare species. The rare component at depth is largely comprised of species more common at other locations near and far. The rare component on the shelf is comprised mostly of species which are consistently rare and restricted in distribution. These observations suggest a shallow–deep difference that is more one of degree than fundamental in nature; the deep having larger regions and regional species pools.

Spatial and temporal variations in the population structure of the soft-shell clam Mya arenaria in the Pomeranian Bay (southern Baltic Sea)

The population structure of Mya arenaria has been investigated in the sublittoral zone of the Pomeranian Bay (southern Baltic Sea). Box-corer samples were collected during a 1.5-year period in 1993/94 to follow changes in size and age structure of the clam populations in different parts of the study area. Large spatial differences in population structure were found between the sheltered southwest of the Bay and the shallow and exposed Oder Bank in the centre. The stock of the Oder Bank consisted of two different clam types. A slow-growing cohort was assumed to be autochtonous on the Oder Bank, whereas a fast-growing one was assumed to have immigrated from the surrounding area. The contribution of the two cohorts to the total density varied seasonally. Because of bedload transport of clams, the contribution to the reduction of the clam stock by wintering sea ducks could not be quantified. In the southwest of the Pomeranian Bay erosion was of minor importance. High mortality rates during the first two years of life were assumed to be caused by predation. Mortality rates of older cohorts remained stable until old age. Variations in cohort strength were related to interannual differences in the reproductive success. A mild winter presumably lowers the reproductive success in the subsequent summer. Severe oxygen depletion in summer 1994 caused a strong reduction in the clam stock at stations deeper than 10 m.

Biogenic and Geochemical Properties of Intertidal Biosedimentary Deposits Related to Mytilus Beds

Abstract. Biofacies development, sedimentology, and geochemical properties of intertidal Mytilus beds in the German Wadden Sea have been studied in vertical profiles in situ and in box core samples. Characteristic features of both the live mussel beds and the underlying sediment sequences were detected. The deposition of faeces, pseudofaeces, and trapped fine‐grained material results in the formation of well‐defined biosedimentary mud layers of several centimetres thickness. The initial stages of this deposition process were observed in a young, growing Myrilus colony over a 17‐month period. In an old mussel bed, being in state of decline, both reworking of dead shells and erosion of the biosedimentary mud layers were recognisable. However, deeper parts of the sediment column reveal shell horizons rich in “pairs” of dead Mytilus shells, i.e., corresponding right and left valves found close together. These represent subrecent records of former Mytilus colonisation. Historically, they point to long‐term occurrence of mussel beds at typical localities, in the framework of fluctuation and patchiness patterns, in the more recent past. On the other hand, mud banks at tidal flat margins adjacent to channels suggest a high preservation potential of biosedimentary mud layers in the low‐water level environment. Geochemical properties of these muddy sediments corroborate their biodepositional origin.

Reassessment of foraminiferal‐based tropical sea surface δ18O paleotemperatures

The possibility exists that the magnitude of Glacial/Holocene δ18O change in the tropical oceans was previously under‐estimated due to the dampening effect of bioturbation on bulk foraminiferal δ18O measurements. We have investigated this possibility by isotopically analyzing suites of individual planktonic foraminifera from box core samples from the equatorial Atlantic. We present oxygen isotopic results for individual specimens of two surface‐dwelling planktonic foraminifera Globigerinoides ruber (white) and Globigerinoides sacculifer (no final sac) from glacial‐age box core samples from seven sites located between about 15°N and 15°S in the tropical Atlantic. These data are used to constrain the mimimum glacial temperatures recorded by tropical surface‐dwelling planktonic foraminifera during the last glacial maximum. The individual foraminiferal δ18O temperatures reveal minimum sea surface temperatures (SSTs) that ranged from 1°C colder to as much as 5° to 6°C colder than present‐day summer SSTs after correcting for an “ice volume effect”. However, these minimum temperatures represent less than 10% of the foraminiferal population measured from glacial horizons. Approximately 80% of the glacial specimens record temperatures that were within 2°C of the mean Holocene SST. We conclude that either the tropical Atlantic was not pervasively colder during the Glacial or that there was a systematic negative shift in the isotopic composition of the surface waters that offset a more signficant temperature change.