Paleoceanographic Proxies Research Articles

Chronostratigraphic framework for the IODP Expedition 318 cores from the Wilkes Land Margin: Constraints for paleoceanographic reconstruction

The Integrated Ocean Drilling Program Expedition 318 to the Wilkes Land margin of Antarctica recovered a sedimentary succession ranging in age from lower Eocene to the Holocene. Excellent stratigraphic control is key to understanding the timing of paleoceanographic events through critical climate intervals. Drill sites recovered the lower and middle Eocene, nearly the entire Oligocene, the Miocene from about 17 Ma, the entire Pliocene and much of the Pleistocene. The paleomagnetic properties are generally suitable for magnetostratigraphic interpretation, with well‐behaved demagnetization diagrams, uniform distribution of declinations, and a clear separation into two inclination modes. Although the sequences were discontinuously recovered with many gaps due to coring, and there are hiatuses from sedimentary and tectonic processes, the magnetostratigraphic patterns are in general readily interpretable. Our interpretations are integrated with the diatom, radiolarian, calcareous nannofossils and dinoflagellate cyst (dinocyst) biostratigraphy. The magnetostratigraphy significantly improves the resolution of the chronostratigraphy, particularly in intervals with poor biostratigraphic control. However, Southern Ocean records with reliable magnetostratigraphies are notably scarce, and the data reported here provide an opportunity for improved calibration of the biostratigraphic records. In particular, we provide a rare magnetostratigraphic calibration for dinocyst biostratigraphy in the Paleogene and a substantially improved diatom calibration for the Pliocene. This paper presents the stratigraphic framework for future paleoceanographic proxy records which are being developed for the Wilkes Land margin cores. It further provides tight constraints on the duration of regional hiatuses inferred from seismic surveys of the region.

Impact of Indus River discharge on productivity and preservation of organic carbon in the Arabian Sea over the twentieth century

Marine high-productivity zones along the continents are of great economic importance, and they account for most organic carbon burial. The carbon cycle in many coastal zones is influenced by the sediments and dissolved nutrients introduced by rivers. However, there is little direct evidence for a regional marine response to changes in river dynamics. Here we present a suite of high-resolution records of organic and inorganic paleoceanographic proxies, which together demonstrate that past changes in Indus River discharge have strongly affected productivity patterns in the coastal northeastern Arabian Sea. Anthropogenic activity, including the building of dams and irrigation facilities during the past century, has drastically decreased the discharge rate of the Indus River. Between A.D. 1890 and 1998, the period over which this reduction occurred, primary productivity off the Pakistan coast seems to have decreased by more than one-third. Over the same period, the regional oxygen minimum zone weakened, increasing the supply of oxygen to the sediments, leaving the expected imprint on a suite of redox-sensitive elements and reducing the preservation of organic matter.

Ba/Ca variations in the modern intertidal bean clam Donax gouldii: An upwelling proxy?

The discovery and calibration of high resolution paleoceanographic proxies is necessary to extend historic climate records and to understand regional climate variability. Chemical variations of skeletal remains have emerged as an often reliable recorder of environmental conditions. Specifically, Ba/Ca ratios have been correlated to temperature, salinity, seawater Ba/Ca, and phytoplankton biomass, although, many of these relationships appear taxon- and location-specific. To assess the sub-weekly Ba/Ca variations in the intertidal shallow-burrowing bivalve Donax gouldii, specimens were collected from the Southern California Bight, skeletal growth increments were cross-dated based on tidal-driven growth patterns, and skeletal aragonite Ba/Ca was determined using laser ablation inductively coupled plasma mass spectrometry. Cross-dated growth among specimens revealed a simultaneous, large, and transient Ba/Ca peak in all shells. The timing of peak Ba/Ca was compared to a suite of locally measured physical and biological data, including temperature, salinity, density, nitrate, silicate, chlorophyll, diatom abundance, dinoflagellate abundance, and phytoplankton community composition. Based on cross-dated chronologies, Ba/Cashell is significantly correlated with Chl a from six and nine days prior and nutrients (nitrate, phosphate, silicate, and nitrite) from three days prior. In this system diatom abundance was not related to Ba/Cashell. Transiently higher seawater Ba/Ca resulting from upwelling may be reflected in peak Ba/Cashell, however the exact mechanisms leading to population wide Ba/Ca peaks remains enigmatic.

Testing the extraction of past seawater Nd isotopic composition from North Atlantic deep sea sediments and foraminifera

Neodymium isotopes provide a paleoceanographic proxy for past deep water circulation and local weathering changes and have been measured on various authigenic marine sediment components, including fish teeth, ferromanganese oxides extracted by acid-reductive leaching, cleaned foraminifera, and foraminifera with Fe-Mn oxide coatings. Here we compare Nd isotopic measurements obtained from ferromanganese oxides leached from bulk sediments and planktonic foraminifera, as well as from oxidatively-reductively cleaned foraminiferal shells from sediment cores in the North Atlantic. Sedimentary volcanic ash contributes a significant fraction of the Nd when the ferro-manganese (Fe-Mn) oxide coatings are leached from bulk sediments. Reductive leachates of marine sediments from North Atlantic core tops near Iceland, or directly downstream from Iceland-Scotland Overflow Waters, record ɛNd values that are significantly higher than seawater, indicating that volcanic material is easily leached by acid-reductive methods. The ɛNd values from sites more distal to Iceland are similar to modern seawater values, showing little contamination from Iceland-derived volcanogenic material. In all comparisons, core top planktonic foraminifera ɛNd values more closely approximate modern deep seawater than the bulk sediment reductive leached value suggesting that the foraminifera provide a route toward quantifying the Nd isotopic signature of deep North Atlantic water masses.

Brachiopod taxa and shell portions reliably recording past ocean environments: Toward establishing a robust paleoceanographic proxy

[1] Fossils of rhynchonelliform brachiopods (marine invertebrates) constitute ∼70% of the samples used for delineating a well-known Phanerozoic trend in oxygen isotope composition (δ18O) of low-Mg calcite shells. The trend represents secular variations in temperature and/or δ18O of ancient seawater. However, the use of brachiopods as a paleoceanographic proxy is based on the presupposition that the shell calcite is precipitated in isotopic equilibrium with ambient seawater. Here, we show that high-resolution time series of the shell δ18O values along the maximum growth axes of two long-lived cool-water brachiopods are identical to, greater than, or less than those of calcite precipitated in isotopic equilibrium with ambient seawater, depending on the difference in shell growth rates. Coupled with δ18O data from subtropical and warm-temperate brachiopod species examined in our previous studies, we provide a sound framework illustrating which taxa and shell portions reliably recorded past ocean environments.

Early Oligocene glaciation and productivity in the eastern equatorial Pacific: Insights into global carbon cycling

The onset of sustained Antarctic glaciation across the Eocene‐Oligocene transition (EOT) marks a pivotal change in Earth's climate, but our understanding of this event, particularly the role of the carbon cycle, is limited. To help address this gap we present the following paleoceanographic proxy records from Ocean Drilling Program Site 1218 in the eastern equatorial Pacific (EEP): (1) stable isotope (δ18O and δ13C) records generated in epifaunal benthic foraminifera (Cibicidoides spp.) to improve (double the resolution) the previously published records; (2) δ18O and δ13C records measured on Oridorsalis umbonatus, a shallow infaunal species; and (3) a record of benthic foraminifera accumulation rate (BFAR). Our new isotope data sets confirm the existence at Site 1218 of a two‐step δ18O increase. They also lend support to the hypothesized existence of a late Eocene transient δ18O increase and early Oligocene Oi‐1a and Oi‐1b glacial maxima. Our record of BFAR indicates a transient (∼500 kyr) twofold to threefold peak relative to baseline Oligocene values associated with the onset of Antarctic glaciation that we attribute to enhanced biological export production in the EEP. This takes the same general form as the history of opal accumulation in the Southern Ocean, suggesting strong high‐to‐low‐latitude oceanic coupling. These findings appear to lend support to the idea that the EOT δ13C excursion is traceable to increased organic carbon (Corg) burial. Paradoxically, early Oligocene sediments in the EEP are extremely Corg‐poor, and proxy records of atmospheric pCO2 indicate a transient increase associated with the EOT.

An ectobiont‐bearing foraminiferan, Bolivina pacifica, that inhabits microxic pore waters: cell‐biological and paleoceanographic insights

The presence of tests (shells) in foraminifera could be taken as an indicator that this protist taxon is unlikely to possess ectosymbionts. Here, however, we describe an association between Bolivina pacifica, a foraminiferan with a calcareous test, and a rod-shaped microbe (bacterium or archaeon) that is directly associated with the pores of the foraminiferan's test. In addition to these putative ectosymbionts, B. pacifica has previously undescribed cytoplasmic plasma membrane invaginations (PMIs). These adaptations (i.e. PMIs, ectobionts), along with the clustering of mitochondria under the pores and at the cell periphery, suggest active exchange between the host and ectobiont. The B. pacifica specimens examined were collected from sediments overlain by oxygen-depleted bottom waters (0.7 μM) of the Santa Barbara Basin (California, USA). An ultrastructural comparison between B. pacifica from the Santa Barbara Basin and a congener (Bolivina cf. B. lanceolata) collected from well-oxygenated sediments (Florida Keys) suggests that PMIs, ectobionts and peripherally distributed mitochondria are all factors that promote inhabitation of microxic environments by B. pacifica. The calcitic δ(13)C signatures of B. pacifica and of a co-occurring congener (B. argentea) that lacks ectobionts differ by > 1.5‰, raising the possibility that the presence of ectobionts can affect incorporation of paleoceanographic proxies.

Response of ostracods to abrupt climate changes in the Western Mediterranean (Gulf of Lions) during the last 30kyr

Abstract Global climate changes attract the attention of researchers working with paleoceanographic proxies for reconstructing past ocean circulation changes and the physico-chemical parameters that control environmental conditions. Moreover, recent studies highlight the importance of climate change on deep sea benthic biodiversity. Deep sea benthic fauna are not isolated from atmospheric processes and climate changes controlling surface ocean biota, for benthic assemblages respond to changing climatic conditions as well. Substrate and physico-chemical characteristics of bottom waters have particularly strong influence on ostracod species distribution in all marine settings, making them excellent environmental indicators. This paper focuses on changes in ostracod assemblages at MD99-2348 site, located on the continental slope of the Gulf of Lions to determine the relationship between climatic-driven oceanographic changes and ostracod diversity and abundance. High abundances of Henryhowella asperrima and Echinocythereis echinata during Heinrich events (HE1 and HE2) may result from increased oxygenation of the bottom water and a more pulsed supply of nutrients (organic matter). Formation of surface cold water that increases the oxygen content in deep layers in the water column is linked to the intensification of northerly wind (Tramontane and Mistral) in northwestern Mediterranean during the acme of HEs, when high latitudes bodies of cold air (Mobile Polar Highs) reach the Mediterranean area. Moreover, the concomitant high abundance of Krithe pernoides , which proved to be very sensitive to variable oxygen levels, probably records even shorter (millennial to centennial scale) climate oscillations. Changes in environmental conditions are illustrated by the shift of C . testudo–Loxoconcha spp. vs. Pseudocythere sp.– Macrocypris sp. that indicates detrital input, primary productivity and the possible impact of sea level oscillation on benthic assemblages since the LGM.

Combined carbonate carbon isotopic and cellular ultrastructural studies of individual benthic foraminifera: Method description

[1] Carbon isotopes of foraminiferal tests provide a widely used proxy for past oceanographic environmental conditions. This proxy can be calibrated using live specimens, which are reliably identified with observations of cell ultrastructure. Observations of ultrastructures can also be used for studies of biological characteristics such as diet and presence of symbionts. Combining biological and isotopic studies on individual foraminifera could provide novel information, but standard isotopic methods destroy ultrastructures by desiccating specimens and observations of ultrastructure require removal of carbonate tests, preventing isotope measurements. The approach described here preserves cellular ultrastructure during isotopic analyses by keeping the foraminifera in an aqueous buffer (Phosphate Buffered Saline (PBS)). The technique was developed and standardized with 36 aliquots of NBS-19 standard of similar weight to foraminiferal tests (5 to 123 μg). Standard errors ranged from ± 0.06 to ± 0.85‰ and were caused by CO2 contaminants dissolved in the PBS. The technique was used to measure δ13C values of 96 foraminifera, 10 of which do not precipitate carbonate tests. Calcareous foraminiferal tests had corrected carbon isotope ratios of −8.5 to +3.2‰. This new technique allows comparisons of isotopic compositions of tests made by foraminifera known to be alive at the time of collection with their biological characteristics such as prey composition and presence or absence of putative symbionts. The approach may be applied to additional biomineralizing organisms such as planktonic foraminifera, pteropods, corals, and coccolithophores to elucidate certain biological controls on their paleoceanographic proxy signatures.

The western North Atlantic record of MIS 13 to 10: Changes in primary productivity, organic carbon accumulation and benthic foraminiferal assemblages in sediments from the Blake Outer Ridge (ODP Site 1058)

We used multiple paleoceanographic proxies to examine sediments from the Blake Outer Ridge (ODP Site 1058, 2996 m water depth) that span the MIS 13–10 time interval. Benthic foraminiferal δ 13C, organic δ 13C, and total δ 15N values are lower in MIS 12 and 10 than in MS 11, and indicate glacial–interglacial changes in both thermohaline circulation and ocean-wide biogeochemical processes that affect deep and surface waters. Three time intervals of higher productivity, as evidenced by higher organic carbon accumulation rates (1.6–4.5 g m − 2 yr − 1 ), occur at the end of MIS 13 and during the early and late parts of MIS 11, in association with the transition from obliquity minima to obliquity maxima. Benthic faunas in these intervals show a distinct increase in opportunistic species that feed on phytodetritus ( Epistominella exigua) or are adapted to elevated but intermittent organic carbon fluxes ( Bulimina aculeata and Bulimina marginata) and that imply episodes of recurrent phytoplankton blooms at the surface and consequent export of labile organic matter to the seafloor. Late interglacial MIS 11.3 is a time of low organic carbon fluxes (< 0.5 g m − 2 yr − 1 ) and is characterized by higher abundances of epifaunal benthic foraminiferal taxa ( Cibicidoides wuellerstorfi, Gyroidina spp., Gyroidinoides spp. and Nuttallides umbonifera) that are typical of modern oligotrophic environments. In contrast, during glacial MIS 12 and 10 higher organic carbon fluxes (> 0.5 g m − 2 yr − 1 ) and benthic assemblages dominated by infaunal taxa ( Uvigerina spp . and Bulimina alazanensis) indicate sustained organic carbon delivery and diminished pore water oxygenation.

The role of seawater endocytosis in the biomineralization process in calcareous foraminifera

Foraminifera are unicellular organisms that inhabit the oceans in various ecosystems. The majority of the foraminifera precipitate calcitic shells and are among the major CaCO(3) producers in the oceans. They comprise an important component of the global carbon cycle and also provide valuable paleoceanographic information based on the relative abundance of stable isotopes and trace elements (proxies) in their shells. Understanding the biomineralization processes in foraminifera is important for predicting their calcification response to ocean acidification and for reliable interpretation of the paleoceanographic proxies. Most models of biomineralization invoke the involvement of membrane ion transporters (channels and pumps) in the delivery of Ca(2+) and other ions to the calcification site. Here we show, in contrast, that in the benthic foraminiferan Amphistegina lobifera, (a shallow water species), transport of seawater via fluid phase endocytosis may account for most of the ions supplied to the calcification site. During their intracellular passage the seawater vacuoles undergo alkalization that elevates the CO(3)(2-) concentration and further enhances their calcifying potential. This mechanism of biomineralization may explain why many calcareous foraminifera can be good recorders of paleoceanographic conditions. It may also explain the sensitivity to ocean acidification that was observed in several planktonic and benthic species.

Southern Ocean evidence for reduced export of North Atlantic Deep Water during Heinrich event 1

Research Article| March 01, 2009 Southern Ocean evidence for reduced export of North Atlantic Deep Water during Heinrich event 1 Laura F. Robinson; Laura F. Robinson 11Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Tina van de Flierdt Tina van de Flierdt 22Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA *Current address: Imperial College London, Department of Earth Science & Engineering, South Kensington Campus, Exhibition Road, London SW7 2AZ, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Laura F. Robinson 11Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Tina van de Flierdt *Current address: Imperial College London, Department of Earth Science & Engineering, South Kensington Campus, Exhibition Road, London SW7 2AZ, UK 22Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA Publisher: Geological Society of America Received: 18 Jul 2008 Revision Received: 08 Oct 2008 Accepted: 14 Oct 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2009 Geological Society of America Geology (2009) 37 (3): 195–198. https://doi.org/10.1130/G25363A.1 Article history Received: 18 Jul 2008 Revision Received: 08 Oct 2008 Accepted: 14 Oct 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Laura F. Robinson, Tina van de Flierdt; Southern Ocean evidence for reduced export of North Atlantic Deep Water during Heinrich event 1. Geology 2009;; 37 (3): 195–198. doi: https://doi.org/10.1130/G25363A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Deep-sea corals form unique, high-resolution archives of ocean circulation that can be dated using the decay of uranium to thorium. They are abundant in the Southern Ocean, and can provide unprecedented insights into ocean circulation and ocean chemistry on sub-millennial time scales in areas where application of traditional paleoceanographic proxies is difficult. Here we present the first coupled neodymium (Nd) isotope and radiocarbon data from deep-sea corals in the Drake Passage (Southern Ocean) adding new constraints on ocean circulation during the last Heinrich event (H1; 16.7 ka). The modern-day Drake Passage water column is homogeneous with respect to Nd isotopes (expressed in epsilon units, ϵNd). Its isotopic value of close to −9 is largely controlled by the mixture of North Atlantic Deep Water and Pacific Deep Water. The aragonite of modern Drake Passage corals reflects this water-column value, whereas a fossil coral from H1 is significantly higher at −6.4 ± 0.4. We interpret this ~2.5 ϵ unit shift as a reduction in the influence of North Atlantic–sourced Nd in the Southern Ocean during H1. This interpretation is supported by a series of radiocarbon analyses on the same sample, and is consistent with a twofold or greater reduction in export of North Atlantic waters from the Atlantic Basin. Combining analyses of radiocarbon and Nd isotopes on U-series dated deep-sea coral skeletons holds great potential for quantification of past ocean ventilation rates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Reconstructing seawater circulation on the Moroccan shelf of Gondwana during the Late Devonian: Evidence from Nd isotope composition of conodonts

The Nd isotope composition of conodonts is used for the first time as a paleoceanographic proxy to reconstruct seawater circulation. Conodonts from two stratigraphic levels in the eastern Anti‐Atlas, the lower part of the Frasnian Zone 11 (base of the Kellwasser facies) and the boundary interval of the Uppermost crepida/rhomboidea zones (top of the Kellwasser facies), were selected for reconstruction of seawater circulation on the Moroccan shelf of Gondwana during Late Devonian times. The recognized lateral variations in ɛNd values are in very good agreement with current‐related sedimentary features. They provide evidence for communication of the Moroccan shelf with the Rheic Ocean to the southwest and with the Variscan Sea to the north. In the Frasnian, the general seawater flow was directed from the southwest to the east and was influenced by a local riverine input of low radiogenic Nd from the south (the West African Craton and/or the Pan‐African Ougarta Belt) and from the northwest (the Precambrian basement of the Anti‐Atlas). The combined set of Nd isotopic records and directional data from the middle Famennian reveals a circulation pattern different from that of the Frasnian. It was dominated by a general westward flow with a countercurrent in the central Tafilalt area. The establishment of a new circulation system during the Famennian suggests essential changes in paleogeography of the western part of the Variscan realm.

Behavior of carbonate-associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy

Although carbonate-associated sulfate (CAS) is used widely as a proxy for the sulfur isotope composition of ancient seawater, little is known about the effects of diagenesis on retention of primary δ 34S signals. Our case study of the Key Largo Limestone, Pleistocene, Florida, is the first systematic assessment of the impact of meteoric diagenesis on CAS properties. Geochemical and petrographic data show that meteoric diagenesis has affected the exposed coralline facies to varying degrees, yielding differences now expressed as sharp reaction fronts between primary and secondary carbonate minerals within individual coral heads. Specifically, analyses across high-resolution transects in the Key Largo Limestone show that concentrations of strontium and sodium decrease across the recrystallization front from original aragonite to meteoric low-magnesium calcite by factors of roughly 5 and 10, respectively. Predictably, δ 18O values decrease across these same fronts. The δ 13C relationships are more complex, with the most depleted values observed in the latest-formed calcite. Such trends likely reflect carbon isotope buffering capacity that decreased as reaction progressed, as well as protracted development of soil profiles and the associated terrestrial biomass and thus depleted δ 13C during sea-level lowstand. Conversely, δ 34S values of CAS vary within a narrow ‘buffered’ range from 20.6 to 22.6‰ (compared to 20.8–22.0‰ of coeval Pleistocene seawater) across the same mineralogical transition, despite sulfate concentrations that drop in the diagenetic calcite by an average factor of 12. Collectively, these data point to robust preservation of primary δ 34S for carbonates that have experienced intense meteoric diagenesis, which is encouraging news for those using the isotopic composition of CAS as a paleoceanographic proxy. At the same time, the vulnerability of CAS concentrations to diagenetic resetting is clear.

Magnetic susceptibility of eastern Mediterranean marine sediments as a proxy for Saharan dust supply?

Interpretations of magnetic susceptibility variations in circum-Saharan marine sediments have suggested a close relationship with Saharan dust supply, which assumes that dust dominates over the potential contributions from a variety of other sources and processes. To evaluate the importance of Saharan dust supply versus that of other potential sources of variability in magnetic susceptibility, we compile magnetic susceptibility data from eastern Mediterranean Plio-Pleistocene sequences at Ocean Drilling Program (ODP) sites 964, 966, 967 and 969, for comparison with other paleoclimatic and paleoceanographic proxy data for the same intervals. Our results demonstrate that magnetic susceptibility variations are linked to the supply of Saharan dust through some parts of the studied intervals, but seem to be predominantly controlled by the discharge of suspended matter from Eurasian rivers and the Nile. Depositional and diagenetic processes in the stratigraphic vicinity of ash layers and sapropels also affect magnetic susceptibility values. We conclude that magnetic susceptibility records can only be used as a proxy for Saharan dust supply in eastern Mediterranean sediments, and likely also in other peri-Saharan marine sediments, when this has been demonstrated by further analyses to be the only (or predominant) source of magnetic susceptibility variability.

Predicting the global distribution of planktonic foraminifera using a dynamic ecosystem model

Abstract. We present a new planktonic foraminifera model developed for the global ocean mixed-layer. The main purpose of the model is to explore the response of planktonic foraminifera to different boundary conditions in the geological past, and to quantify the seasonal bias in foraminifera-based paleoceanographic proxy records. This model is forced with hydrographic data and with biological information taken from an ecosystem model to predict monthly concentrations of the most common planktonic foraminifera species used in paleoceanography: N. pachyderma (sinistral and dextral varieties), G. bulloides, G. ruber (white variety) and G. sacculifer. The sensitivity of each species with respect to temperature (optimal temperature and range of tolerance) is derived from previous sediment-trap studies. Overall, the spatial distribution patterns of most of the species are in agreement with core-top data. N. pachyderma (sin.) is limited to polar regions, N. pachyderma (dex.) and G. bulloides are the most common species in high productivity zones, while G. ruber and G. sacculifer are more abundant in tropical and subtropical oligotrophic waters. For N. pachyderma (sin) and N. pachyderma (dex.), the season of maximum production coincides with that observed in sediment-trap records. Model and sediment-trap data for G. ruber and G. sacculifer show, in general, lower concentrations and less seasonal variability at all sites. A sensitivity experiment suggest that, within the temperature-tolerance range of a species, food availability may be the main parameter controlling its abundance.

Sediment dynamics in the Eurasian Arctic Ocean during the last deglaciation — The clay mineral group smectite perspective

This study focuses on sedimentological investigations of sediment cores recovered during the international Arctic′91, expeditions with the German research ice breaker RV “ Polarstern” to the European sector of the Arctic Ocean. Here, we deduce the last glacial/interglacial changes in transport mechanism and sedimentation from the clay mineral group smectite. We choose the smectites as an example of how sediment mineralogy can be linked with particular source regions (the Kara and Laptev seas), distinct transport mechanism (sea ice and surface currents) and sedimentation processes. Smectite contents in Arctic sediments discussed for two time slices, including the Last Glacial Maximum (LGM), and the last deglaciation (Termination I), reveal the highest variability subsequent to the retreat of the Eurasian ice sheets. Our results show that smectite anomalies in the Eurasian Basin are associated with distinct meltwater pulses and occurred around 13.5–13.0 14C ka B.P. Compelling evidence is provided that these anomalies are deduced from sea-ice entrained sediments from the eastern Kara Sea that entered the Arctic Ocean after ice-sheet break-up and eventually flooding of the Kara Sea. We propose that smectite anomalies in sediments of the eastern Arctic Ocean can be utilized to identify deglacial events and to help decipher configurations of the Eurasian ice sheets. The identification of smectite maxima along the modern sea-ice edge in the Eurasian Basin further indicates biologically enhanced sedimentation from melting sea ice allowing the reconstruction of seasonally open water in the region. Hence, considering the poor preservation conditions of primary paleoceanographic proxies in the Arctic Ocean, the clay mineral contents, particularly the smectite group, may be one alternative tool for paleoclimatic reconstruction in the Eurasian Basin.

Varying growth rates in bamboo corals: sclerochronology and radiocarbon dating of a mid-Holocene deep-water gorgonian skeleton (Keratoisis sp.: Octocorallia) from Chatham Rise (New Zealand)

A branched mid-Holocene bamboo coral skeleton of the isidid gorgonian genus Keratoisis (Octocorallia) recovered at southwestern Chatham Rise (New Zealand) from an average water depth of 680 m is described with respect to sclerochronology and age determination. Growth rates of the Mg-calcitic internodal increments were investigated by the counting of colour bands and radiocarbon dating. Growth banding is produced by varying orientations of crystal fan bundles towards the image plane. The skeleton shows three growth interruptions, which are documented in all branches. AMS 14C ages decrease from base to top of the trunk and from the central axes to the margins of the branches, documenting a simultaneous vertical and lateral growth. The data provide a maximum age of 3,975 ± 35 years BP, and a record spanning 240 ± 35 years. While calculated longitudinal growth rates amount to an average of 5 mm year−1 during a 55-year record, average lateral linear extension rates of 0.4 mm year−1 are an order of magnitude lower, still allowing for a seasonal to annual resolution of colour bands on a macroscopic scale and for a daily to monthly resolution on microscales of individual crystal generations to fascicle bundles. Hence, the isidid skeleton provides a high-resolution archive of paleoceanographic dynamics in deeper water masses. Concentric incremental accretion around the central axis in the early growth stages changed into a unilaterally asymmetric growth during late-stage evolution, probably triggered by the establishment of a stable system of unidirectional currents and nutrient flux. While colour band counts, related to the AMS 14C ages, support a seasonal to annual accretion of macroscopic growth bands in the inner concentric and complete outer parts of the skeleton, incremental growth rates at the condensed side are highly variable, as documented by hiatuses and unconformities. Thus the specimen proves that growth rates of bamboo corals may vary within individual skeletons and strongly deviate from the annual mode, hence showing implications on paleoceanographic proxy analyses.

Experimental determination of salinity, temperature, growth, and metabolic effects on shell isotope chemistry ofMytilus eduliscollected from Maine and Greenland

To study the effects of temperature, salinity, and life processes (growth rates, size, metabolic effects, and physiological/genetic effects) on newly precipitated bivalve carbonate, we quantified shell isotopic chemistry of adult and juvenile animals of the intertidal bivalveMytilus edulis(Blue mussel) collected alive from western Greenland and the central Gulf of Maine and cultured them under controlled conditions. Data for juvenile and adultM. edulisbivalves cultured in this study, and previously by Wanamaker et al. (2006), yielded statistically identical paleotemperature relationships. On the basis of these experiments we have developed a species‐specific paleotemperature equation for the bivalveM. edulis[T °C = 16.28 (±0.10) − 4.57 (±0.15) {δ18OcVPBD −δ18OwVSMOW} + 0.06 (±0.06) {δ18OcVPBD −δ18OwVSMOW}2; r2= 0.99; N = 323; p &lt; 0.0001]. Compared to the Kim and O'Neil (1997) inorganic calcite equation,M. edulisdeposits its shell in isotope equilibrium (δ18Ocalcite) with ambient water. Carbon isotopes (δ13Ccalcite) from sampled shells were substantially more negative than predicted values, indicating an uptake of metabolic carbon into shell carbonate, andδ13Ccalcitedisequilibrium increased with increasing salinity. Sampled shells ofM. edulisshowed no significant trends inδ18Ocalcitebased on size, cultured growth rates, or geographic collection location, suggesting that vital effects do not affectδ18OcalciteinM. edulis. The broad modern and paleogeographic distribution of this bivalve, its abundance during the Holocene, and the lack of an intraspecies physiologic isotope effect demonstrated here make it an ideal nearshore paleoceanographic proxy throughout much of the North Atlantic Ocean.

Metal Stable Isotopes in Paleoceanography

Considered esoteric only a few years ago, research into the stable isotope geochemistry of transition metals is moving into the geoscience mainstream. Although initial attention focused on the potential use of some of these nontraditional isotope systems as biosignatures, they are now emerging as powerful paleoceanographic proxies. In particular, the Fe and Mo isotope systems are providing information about changes in oxygenation and metal cycling in ancient oceans. Zn, Cu, Tl, and a number of other metals and metalloids also show promise. Here we review the basis of stable isotope fractionation as it applies to these elements, analytical considerations, and the current status and future prospects of this rapidly developing research area.