Paleoceanographic Tracers Research Articles

Glacial state of the global carbon cycle: time-slice simulations for the last glacial maximum with an Earth-system model

Abstract. Three time-slice carbon cycle simulations for the last glacial maximum (LGM) constrained by the CO2 concentration in the atmosphere and the increase in the mean concentration of dissolved inorganic carbon in the deep ocean were carried out with a fully coupled comprehensive climate model (the Community Earth System Model version 1.2). The three modelled LGM ocean states yielded different physical features in response to artificial freshwater forcing, and, depending on the physical states, suitable amounts of carbon and alkalinity were added to the ocean to satisfy constraints from paleo-data. In all the simulations, the amount of carbon added was in line with the inferred transfers of carbon among various reservoirs during the evolution from the LGM to the pre-industrial (PI) period, suggesting that the simulated glacial ocean states are compatible with the PI one in terms of the carbon budget. The increase in total alkalinity required to simulate ocean states that were deemed appropriate for the LGM was in broad quantitative accord with the scenario of post-glacial shallow water deposition of calcium carbonate, although a more precise assessment would demand further studies of various processes such as the land chemical weathering and deep-sea burial of calcium carbonates, which have affected the alkalinity budget throughout history since the LGM. On the other hand, comparisons between the simulated distributions of paleoceanographic tracers and corresponding reconstructions clearly highlighted the different water-mass geometries and favoured a shallower Atlantic meridional overturning circulation (AMOC) for the LGM as compared to PI.

Radiocarbon as a Dating Tool and Tracer in Paleoceanography

AbstractRadiocarbon is an extremely useful carbon cycle tracer and radiometric dating tool. Here, we review the main principles and challenges involved in the use of radiocarbon in paleoceanography. First, we present a conceptual framework in which there are three possible uses of a radiocarbon measurement: (a) to obtain a calendar age interval, or a fossil entity's age; (b) to obtain an estimate of a carbon reservoir's past radiocarbon activity; or (c) to compare the relative radiocarbon activities of two contemporary carbon reservoirs. We discuss the analysis of marine fossil material, the generation of an atmospheric reference curve, and the interpretation of marine radiocarbon “ventilation metrics” in relation to this reference curve. It is emphasized that marine radiocarbon integrates the influences of: changing radiocarbon production; air‐sea gas exchange effects at the sea surface; transport times within the ocean interior; and the mixing of water parcels with different transit times from the sea surface, and different sea‐surface sources. These controls are what make radiocarbon such a powerful paleoceanographic tracer, though the difficulty of disentangling them is what makes marine radiocarbon dating and tracer studies so challenging. We discuss the implementation of radiocarbon in numerical models, and explore the theory linking ocean‐atmosphere partitioning of radiocarbon and CO2. Finally, we review existing records of marine radiocarbon variability over the last ∼25,000 years, which highlight the influence of ocean‐atmosphere carbon exchange on past atmospheric CO2 and climate, and point to emerging opportunities for resolving the global radiocarbon‐ and carbon budgets over the last glacial cycle.

Carbonic anhydrase, coral calcification and a new model of stable isotope vital effects

The stable isotope compositions of biogenic carbonates have been used for paleoceanographic and paleoclimatic reconstructions for decades, and produced some of the most iconic records in the field. However, we still lack a fully mechanistic understanding of the stable isotope proxies, especially the biological overprint on the environmental signals termed “vital effects”. A ubiquitous feature of stable isotope vital effects in marine calcifying organisms is a strong correlation between δ18O and δ13C in a range of values that are depleted from inorganic calcite/aragonite. Two mechanisms have been proposed to explain this correlation, one based on kinetic isotope effects during CO2(aq)-HCO3− inter-conversion, the other based on equilibrium isotope exchange during pH dependent speciation of the dissolved inorganic carbon (DIC) pool. Neither mechanism explains all the stable isotope features observed in biogenic carbonates. Here we present a fully kinetic model of biomineralization and its isotope effects using deep-sea corals as a test organism. A key component of our model is the consideration of the enzyme carbonic anhydrase in catalyzing the CO2(aq)-HCO3− inter-conversion reactions in the extracellular calcifying fluid (ECF). We find that the amount of carbonic anhydrase not only modulates the carbonate chemistry of the calcifying fluid, but also helps explain the slope of the δ18O-δ13C correlation. Differences in CA activity in the biomineralization process can possibly explain the observed range of δ18O-δ13C slopes in different calcifying organisms. A mechanistic understanding of stable isotope vital effects with numerical models can help us develop better paleoceanographic tracers.

Enrichment of dissolved silica in the deep equatorial Pacific during the Eocene‐Oligocene

AbstractSilicon isotope ratios (expressed as δ30Si) in marine microfossils can provide insights into silica cycling over geologic time. Here we used δ30Si of sponge spicules and radiolarian tests from the Paleogene Equatorial Transect (Ocean Drilling Program Leg 199) spanning the Eocene and Oligocene (~50–23 Ma) to reconstruct dissolved silica (DSi) concentrations in deep waters and to examine upper ocean δ30Si. The δ30Si values range from −3.16 to +0.18‰ and from −0.07 to +1.42‰ for the sponge and radiolarian records, respectively. Both records show a transition toward lower δ30Si values around 37 Ma. The shift in radiolarian δ30Si is interpreted as a consequence of changes in the δ30Si of source DSi to the region. The decrease in sponge δ30Si is interpreted as a transition from low DSi concentrations to higher DSi concentrations, most likely related to the shift toward a solely Southern Ocean source of deep water in the Pacific during the Paleogene that has been suggested by results from paleoceanographic tracers such as neodymium and carbon isotopes. Sponge δ30Si provides relatively direct information about the nutrient content of deep water and is a useful complement to other tracers of deep water circulation in the oceans of the past.

Temporal record of osmium concentrations and 187Os/188Os in organic-rich mudrocks: Implications for the osmium geochemical cycle and the use of osmium as a paleoceanographic tracer

We present a compilation of 192Os concentrations (representing non-radiogenic Os) and initial 187Os/188Os isotope ratios from organic-rich mudrocks (ORM) to explore the evolution of the Os geochemical cycle during the past three billion years. The initial 187Os/188Os isotope ratio of a Re-Os isochron regression for ORM constrains the local paleo-seawater 187Os/188Os, which is governed by the relative magnitudes of radiogenic Os (old continental crust) and unradiogenic Os (mantle, extraterrestrial, and juvenile/mafic/ultramafic crust) fluxes to seawater. A first-order increase in seawater 187Os/188Os ratios occurs from the Archean to the Phanerozoic, and may reflect a combination of increasing atmosphere-ocean oxygenation and weathering of progressively more radiogenic continental crust due to in-growth of 187Os from radioactive decay of 187Re. Superimposed on this long-term trend are shorter-term fluctuations in seawater 187Os/188Os ratios as a result of climate change, emplacement of large igneous provinces, bolide impacts, tectonic events, changes in seafloor spreading rates, and lithological changes in crustal terranes proximal to sites of ORM deposition. Ediacaran-Phanerozoic ORM have mildly higher 192Os concentrations overall compared with pre-Ediacaran Proterozoic ORM based on the mean and 95% confidence interval of 10,000 median values derived using a bootstrap analysis for each time bin (insufficient Archean data exist for robust statistical comparisons). However, there are two groups with anomalously high 192Os concentrations that are distinguished by their initial 187Os/188Os isotope ratios. Ediacaran-Cambrian ORM from South China have radiogenic initial 187Os/188Os, suggesting their high 192Os concentrations reflect proximal Os-rich crustal source(s), ultraslow sedimentation rates, and/or other unusual depositional conditions. In contrast, the unradiogenic initial 187Os/188Os and high 192Os concentrations of some Mesozoic ORM can be tied to emplacement of large igneous provinces. Excluding these two anomalous groups and repeating the bootstrap analysis, we find that, overall, the 192Os concentrations for the Ediacaran-Phanerozoic and pre-Ediacaran Proterozoic time bins are not significantly different. An improved understanding of Os geochemical behavior in modern environments is required before our compilation can be fully used to constrain the temporal evolution of the seawater Os reservoir.

Comparative geochemistry of four ferromanganese crusts from the Pacific Ocean and significance for the use of Ni isotopes as paleoceanographic tracers

Ferromanganese (Fe-Mn) crusts are potential archive of the Ni isotope composition of seawater through time. In this study we aim at (1) understanding Ni isotope fractionation mechanisms and metal enrichment processes in Fe-Mn deposits, (2) addressing global vs. local control of Ni isotope composition of these deposits. Two Fe-Mn crusts from the North Pacific Ocean (Apuupuu Seamount, Hawaii) and two Fe-Mn crusts from the South Pacific Ocean (near Rurutu Island, Austral archipelago of French Polynesia) were characterized for their elemental geochemistry and Ni isotope composition. Geochemical analyses were performed at millimeter intervals in order to provide time-resolved record of Ni isotopes. Chronology and growth rates were determined using cosmogenic 10Be isotope abundances. The results show that, despite different growth rates, textures and geochemical patterns, Fe-Mn crusts from both North and South Pacific Oceans have fairly homogenous Ni isotope compositions over the last ∼17Ma, yielding average δ60/58Ni values of 1.79±0.21‰ (2sd, n=31) and 1.73±0.21‰ (2sd, n=21) respectively. In one crust sample, however, layers directly in contact with the altered substrate show anomalously light δ60/58Ni values down to 0.25±0.05‰ (2se) together with rejuvenated 10Be/9Be ratios correlating with elevated Ni/Mn ratios. Such patterns are best explained by protracted fluid–rock interactions leading to alteration of Mn-phases after crust formation. Isotopically light Ni would be the result of Ni isotope fractionation during adsorption rather than the contribution of external Ni sources (e.g. hydrothermal sources) having light Ni isotope compositions. The combination of our results with previously published data on Fe-Mn crusts indicates that the average Ni isotope composition in deep waters has not changed through the Cenozoic (∼70Ma). We propose that Ni isotope variations in Fe-Mn crusts may not only record variations of Ni sources to the oceans, but also post-depositional processes depending on the growth history and geological settings of Fe-Mn crusts.

The structure of benthic environments and the paleochemical record of foraminifera

Benthic foraminiferal shell geochemistry has been extensively used to develop paleoceanographic tracers. Many of these proxies are sensitive to the geochemical conditions of the microhabitats selected by particular foraminiferal species. Understanding these microhabitats, then, is essential for proper interpretation of the proxies. A simple, broadly accepted, view is that foraminiferal species’ habitats are vertically stratified in the sediments due to general pore-water chemical gradients, which develop in response to the seabed organic carbon flux. Species are categorized into epifaunal, shallow infaunal and deep infaunal habitats, and are supposed to acquire the geochemical characteristics of these. However, this view is at odds with species’ distributional data and foraminiferal geochemical properties. We present an alternate model in which foraminifera select for habitats within the bio-irrigation system of the sediments created by the activities of macro-/meio-fauna. Our distributional and geochemical data indicate that foraminiferal species seek particular biotic associations and geochemical conditions within the complex bio-architecture of the sediments and are not tied to particular sediment depths, or the general pore-water chemistry of their apparent habitation zone. Instead, foraminifera inhabit micro-environments with steep oxic to anoxic gradients. This might account for disparities among geochemical tracers.

Combined carbonate carbon isotopic and cellular ultrastructural studies of individual benthic foraminifera: 2. Toward an understanding of apparent disequilibrium in hydrocarbon seeps

[1] Numerous previous studies show disequilibrium between stable carbon isotope ratios of foraminiferal calcite and pore water dissolved inorganic carbon in hydrocarbon seeps, calling into question the utility of this widely used paleoceanographic tracer as a proxy. We use a recently developed method to compare stable carbon isotope ratios of foraminiferal carbonate with cell ultrastructural observations from individual benthic foraminifera from seep (under chemosynthetic bivalves) and nonseep habitats in Monterey Bay, California, to better understand control(s) of benthic foraminiferal carbon isotope ratios. Two attributes previously proposed to cause the isotopic offsets are diet and symbionts. Ultrastructural analysis shows that positive staining with Rose Bengal indicates presence of foraminiferal cytoplasm, bacterial biomass, or a combination of both and, thus, is not an unequivocal indicator of viability. We also show for the first time that some living seep foraminifera have endobionts. Results from our unique, yet limited, data set are consistent with suggestions that, in our sites, several foraminiferal species collected from seep clam beds may not survive there, diet and symbiont presence do not appear to be major contributors to disequilibrium, little calcification of seep-tolerant foraminiferal species occurs while seep conditions prevail, and microscale variability in habitats could influence δ13C of benthic foraminiferal carbonate. Results further suggest that our knowledge of benthic foraminiferal ecology and biomineralization, especially in extreme habitats such as seeps, must be bolstered before we fully understand the fidelity of paleoenvironmental records derived from benthic foraminiferal test δ13C data.

Plate tectonics, radiogenic isotopic tracers and paleoceanography: The case of the manganese crusts in the Pacific

Hydrogenous ferromanganese crust deposits on seafloor and volcanic seamounts are archives of past isotopic compositions of the ocean. Isotopic compositions of long-life radiogenic isotopes that are dissolved in the ocean such as Nd and Pb, are mixtures of different erosion products (continents, volcanics arcs, ridges etc…) transported by oceanic currents or winds. Isotopic studies on Mn crusts are thus potential paleogeographic and paleoceanographic tracers. However, global paleogeography can drastically evolve over the period of growth of crust, as well as the latitude, longitude and water depth of the Mn crust. Thus each crust sampled a succession of geographic sites related to its spatial evolution through time. For this reason, valid information about the past ocean isotopic composition can only be extracted from Mn crust under the condition that the successive paleogeographic and paleodepth positions can be reconstructed for each Mn crust. We followed this approach for six hydrogenous Mn crusts from the Pacific Ocean that have recorded Pb and Nd isotopic signals for the past 80 Myr. The evolution of the isotopic data was considered within a proper geodynamical framework that takes into account the paleopositions of the Pacific plate and surrounding continents, of the hot spots and ridges as well as the paths of the six crusts. The differences and similarities between the Pb and Nd revealed by our analysis emphasize their complementarity as paleoceanographic tracers. The Nd isotopes are strongly influenced by the exchange between Indian and Pacific waters, specifically by the progressive closure of the Indonesian Gateway. The closed central Pacific characteristics are defined from the extrapolation of the isotopic trend. This highlights the important role played by erosion of volcanic arcs in the Nd isotopic value of seawater. Our approach also stresses the influence of local volcanic activity (ridges or active hot spot) for lead isotopes.

Can Paleoceanographic Tracers Constrain Meridional Circulation Rates?

Abstract The ability of paleoceanographic tracers to constrain rates of transport is examined using an inverse method to combine idealized observations with a geostrophic model. Considered are the spatial distribution, accuracy, and types of tracers required to constrain changes in meridional transport within an idealized single-hemisphere basin. Measurements of density and radioactive tracers each act to constrain rates of transport. Conservative tracers, while not of themselves able to inform regarding rates of transport, improve constraints when coupled with density or radioactive observations. It is found that the tracer data would require an accuracy one order of magnitude better than is presently available for paleo-observations to conclusively rule out factor-of-2 changes in meridional transport, even when assumed available over the entire model domain. When data are available only at the margins and bottom of the model, radiocarbon is unable to constrain transport while density remains effective only when a reference velocity level is assumed. The difficulty in constraining the circulation in this idealized model indicates that placing firm bounds on past meridional transport rates will prove challenging.

Impact of paleocirculations on the silicon redistribution in the world ocean

A global biogeochemical ocean general circulation model with a representation of the marine silicon (Si) cycle including the bioturbated sediment zone is integrated into full equilibrium including for four different velocity fields: One preindustrial circulation under partial use of forcing data from an atmospheric model, one preindustrial circulation under use of climatological atmospheric forcing fields from measurements only, a circulation representing Last Glacial Maximum conditions, and one circulation reflecting the mid-Miocene ocean with an open Panama isthmus. Results of paleo-simulations are compared with a paleoceanographic sediment data base for opal. Marine Si budgets for the glacial and pseudo mid-Miocene oceans are presented under the same biogeochemical forcing. It turns out that the sediment distribution reacts sensitively to a switch between velocity fields while the overall budgets and fluxes between Si cycle compartments change only to a minor degree. This indicates that opal sediment has a considerable potential as a paleoceanographic tracer for ocean circulation and biological productivity.

Ba, Ra, Th, and U in marine mollusc shells and the potential of 226Ra/Ba dating of Holocene marine carbonate shells

The geochemistry of Ba, Ra, Th, and U and the potential of using 226Ra/Ba ratios as an alternative dating method are explored in modern and Holocene marine mollusc shells. Five modern shells of the Antarctic scallop Adamussium colbecki collected from the present day beach and six radiocarbon dated specimens from Holocene beach terraces of the Ross Sea region (Antarctic) between 700 and 6100 calibrated yr BP old have been analysed by mass spectrometry. In clean shells 226Ra concentrations and 226Ra/Ba ratios show a clear decrease with increasing age, suggesting the possibility of 226Ra dating. Limiting factors for such dating are Ba and 226Ra present in surface contaminants, and ingrowth of 226Ra from U present within the shell. Surface contamination is difficult to clean off entirely, but moderate levels of residual contamination can be corrected using 232Th. Sub-samples from the same shell with different proportions of contamination form a mixing line in a 226Ra/Ba– 232Th/Ba graph, and the 226Ra/Ba of the pure shell can be derived from the intercept on the 226Ra/Ba axis. Contaminant corrected 226Ra/Ba ratios of late-Holocene 14C-dated samples fall close to that expected from simple 226Ra excess decay from seawater 226Ra/Ba values. 226Ra ingrowth from U incorporated into the shell during the lifetime of the mollusc can be corrected for. However, the unknown timing of post mortem U uptake into the shell makes a correction for 226Ra ingrowth from secondary U difficult to achieve. In the A. colbecki shells, 226Ra ingrowth from such secondary U becomes significant only when ages exceed ∼2500 yr. In younger shells, 226Ra/Ba ratios corrected for surface contamination provide chronological information. If evidence for a constant oceanic relationship between 226Ra and Ba in the ocean can be confirmed for that time scale, the 226Ra/Ba chronometer may enable the reconstruction of variability in sea surface 14C reservoir ages from mollusc shells and allow its use as a paleoceanographic tracer.

Adaptation of a flow‐through leaching procedure for Mg/Ca paleothermometry

Mg/Ca ratios in planktonic foraminifera reflect calcification temperatures and are thus useful for sea surface temperature (SST) reconstructions. Despite the obvious utility of this paleoceanographic tracer, problems of dissolution, gametogenic calcification, and contaminant phases have thus far limited confidence in Mg/Ca‐based reconstructions. Here we show strong evidence of Mg heterogeneity in foraminiferal calcite by sequentially measuring the composition of different forms of calcite (ontogenetic, gametogenic, diagenetic) in the same shells, while monitoring and removing contaminant phases. A new flow‐through method combines chromatographic technology and inductively coupled plasma mass spectrometry (ICP‐MS) in a series of cleaning and dissolution reactions monitored continuously with time‐resolved analysis (TRA). This combination of slow, controlled dissolution and TRA provides a complete elemental description of contaminant phases and sorts the cleaned calcium carbonate based on dissolution sensitivity. Examination of partially dissolved shells with electron microscopy suggests that the flow‐through method simulates the natural dissolution sequence and effectively separates the different calcite domains within a single foraminiferal shell. Heterogeneity of Mg/Ca in foraminiferal calcite is clearly demonstrated in flow‐through analysis. Foraminiferal shells have initially high Mg levels that decrease steadily throughout dissolution. Later dissolution yields lower Mg/Ca, which is likely due to a combination of subsurface calcification and biomineralization effects. Mg/Ca ratios from the most dissolution‐sensitive (high‐Mg) portions of surface‐dwelling species in core tops are used to calculate calcification temperatures. A comparison of late Holocene core top data with World Ocean Atlas SST data indicates that the flow‐through method does yield viable SST estimates. Furthermore, a depth transect in the eastern tropical Pacific suggests that this approach provides the opportunity to extract initial calcification temperatures despite partial dissolution of foraminiferal shells.

Measurement of silicon stable isotope natural abundances via multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS)

Widespread utilization of silicon isotope ratio variations (δ29Si and δ30Si) as a geochemical and paleoceanographic tracer has been hampered by the hazardous nature of the analysis, which requires the use of a fluorinating gas. Multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS) provides a safer means of silicon isotope analysis. High backgrounds, long rinsing and sample introduction times, and interferences are significant difficulties. It is possible to work around these obstacles, and the measurement of δ29Si may be done to a precision averaging ± 0.1‰. The δ30Si, which cannot be measured directly due to a NO interference at mass 30, may be extrapolated from the theoretically derived and empirically verified relationship, δ30Si = 1.93δ29Si. Biological materials (sponge spicules and diatoms) measured via MC‐ICP‐MS yielded δ30Si values in the ranges expected from the prior analysis of similar samples by the method of fluorination.

RADIOGENIC ISOTOPES: TRACERS OF PAST OCEAN CIRCULATION AND EROSIONAL INPUT

The radiogenic isotope composition of dissolved trace metals in the ocean represents a set of relatively new and not yet fully exploited tracers with a large potential for oceanographic and paleoceanographic research on timescales from the present back to at least 60 Ma. The main topic of this review are those trace metals with oceanic residence times on the order of or shorter than the global mixing time of the ocean (Nd, Pb, Hf, and, in addition, Be). Their isotopic composition in the ocean has varied as a function of changes in paleocirculation, source provenances, style and intensity of weathering on the continents, as well as orogenic processes. The relative importance of these processes for each trace metal is evaluated, which is a prerequisite for reliable interpretation of their time series in terms of changes in paleocirculation or weathering inputs. This analysis of processes includes a discussion of the long‐term isotopic evolution of Sr and Os, which are well mixed in the ocean and have thus not been influenced by circulation changes. The radiogenic isotope evolution of those trace metals with intermediate oceanic residence times can be used as paleoceanographic proxies to reconstruct paleocirculation and weathering inputs into the ocean. This is demonstrated by studies from different ocean basins, mainly carried out on ferromanganese crusts, which show that radiogenic trace metal isotopes provide important new insights and can complement results obtained by other well‐established paleoceanographic tracers such as carbon isotopes.

Zinc concentrations in benthic foraminifera reflect seawater chemistry

We have measured Zn/Ca ratios in two taxa of Holocene‐aged benthic foraminifera from throughout the world's oceans. Zn/Ca is controlled by bottom water dissolved Zn concentration and, like Cd/Ca and Ba/Ca, by bottom water saturation state with respect to calcite. Measurements on “live‐collected” foraminifera suggest that the saturation effect occurs during growth and is not a postdepositional artifact. Zn/Ca could be a sensitive paleoceanographic tracer because deep water masses have characteristic Zn concentrations that increase about tenfold from the deep North Atlantic to the deep North Pacific. In addition, since Zn/Ca responds to a different range of saturation states than Cd/Ca, the two may be used together to evaluate changes in deep water carbonate ion (CO32−) concentration.

Carbon isotopic disequilibrium in polar planktonic foraminifera and its impact on modern and Last Glacial Maximum reconstructions

Calibration studies of Neogloboquadrina pachyderma (sinistral coiling, hereinafter referred to as N. pachyderma (s.)) in the South Atlantic raise questions about the interpretation of this species' carbon isotope composition as a paleoceanographic tracer of Southern Ocean surface nutrient contents. Carbon isotope disequilibrium between δ13C of total CO2 (TCO2) of seawater and δ13C of N. pachyderma (s.) increases systematically between 70° and 40°S. Several effects could cause the observed carbon isotope disequilibrium, including a combination of foraminiferal diet, calcification temperature, and carbonate ion chemistry. Combining these corrections to the δ13C of N. pachyderma (s.) improves the reconstruction of the δ13C of seawater TCO2 between 0 and 200 m in the modern South Atlantic. With these corrections applied, the δ13C for equilibrium calcite reconstructed from N. pachyderma (s.) at the Last Glacial Maximum is very similar to preindustrial values, suggesting that one cannot rule out the possibility that surface nutrient concentrations in the South Atlantic Ocean were comparable to today. However, the magnitude of the uncertainties associated with these corrections make it difficult to assess absolute paleonutrient concentrations with much confidence.

Deep-sea ostracode species diversity: response to late Quaternary climate change

Late Quaternary ostracode assemblages from the North Atlantic Ocean were studied to establish the effect of climatic changes of the past 210,000 yr (marine oxygen isotope stages 7–1) on deep-sea benthic biodiversity and faunal composition. Two-hundred and twenty five samples from the Chain 82-24 Core 4PC (41°43′N, 32°51′W, 3427 m water depth) on the western Mid-Atlantic Ridge revealed high amplitude fluctuations in ostracode abundance and diversity coincident with orbital and suborbital scale climate oscillations measured by several paleoceanographic proxy records. During the past 210,000 yr, ostracode biodiversity as measured by species number ( S) and the Shannon–Weaver index, H( S), oscillated from H( S)=0.4 during glacial periods (marine isotope stages 6, 5d, 5b, 4, and 2) to H( S)=1.1 during interglacial and interstadial periods (stages 7, 5e, 5c, 5a, 3 and 1). A total of 23 diversity peaks could be recognized. Eleven of these signify major periods of high diversity [ H( S)>0.8, S = 10–21] occurring every 15–20 ka. Twelve were minor peaks which may represent millennial-scale diversity oscillations. The composition of ostracode assemblages varies with Krithe-dominated assemblages characterizing glacial intervals, and Argilloecia–Cytheropteron characterizing deglacials, and trachyleberid genera ( Poseidonamicus, Echinocythereis, Henryhowella, Oxycythereis) abundant during interglacials. Diversity and faunal composition changes can be matched to independent deep-sea paleoceanographic tracers such as benthic foraminiferal carbon isotopes, Krithe trace elements (Mg/Ca ratios), and to North Atlantic region climate records such as Greenland ice cores. When interpreted in light of ostracode species' ecology, these faunal and diversity patterns provide evidence that deep-sea benthic ecosystems experience significant reorganization in response to climate changes over orbital to millennial timescales.

Oxygen isotope evolution of Phanerozoic seawater

Abstract A compilation of over 2000 measurements of 18O and 13C on Phanerozoic low-Mg calcite shells, such as brachiopods, belemnites and oysters, delineates secular 18O/16O and 13C/12C variations that are similar to those previously described for whole rocks. The trend for the δ18O suggests about ∼5±2‰ enrichment from the Cambrian to today. In contrast, the δ13C rise during the Paleozoic is followed by its decline in the Mesozoic and Cenozoic. Optical (textural) and chemical criteria suggest that the interior “secondary” layer of the brachiopod shells, the material that carries these signals, is well preserved in many samples and the extracted secular isotopic trends are therefore a primary feature of the geologic record. The similarity of the δ18O/δ13C isotope patterns in ancient and modern brachiopods also supports such an interpretation. In our view, the 18O enrichment in progressively younger samples is principally, although not exclusively, a reflection of the evolving 18O/16O composition of seawater. If so, a delineation of this trend may ultimately result in development of a valuable paleoclimatic and paleoceanographic tracer for the Phanerozoic.

Neogloboquadrina pachyderma (sinistral coiling) as paleoceanographic tracers in polar oceans: Evidence from northeast water polynya plankton tows, sediment traps, and surface sediments

The abundance and chemistry of the planktonic foraminifera Neogloboquadrina pachyderma (sinistral coiling) have long been used as tools for monitoring polar surface ocean changes and for correlating these changes to atmospheric and thermohaline circulation fluctuations. However, due to its remote habitat, very little is known about how modern N. pachyderma (s.) respond to changing environmental conditions in the polar seas. Modern samples of N. pachyderma (s.) from the Northeast Water Polynya provide a means for studying how environmental conditions affect the vertical distribution and chemistry of this species. Highest abundances of N. pachyderma (s.) were associated with the chlorophyll maximum in the surface 20–80 m, where they are exploiting their primary food source. Evidence suggests that the addition of a calcite crust modifies the calcite tests of some N. pachyderma (s.) between 50 and 200 m, increasing shell density and modifying shell chemistry. The shell mass of encrusted forms is 3–4 times greater than the nonencrusted forms between 50 and 200 m. The oxygen isotope composition of N. pachyderma (s.) shells increase by 1.5‰ in response to local water column gradients. The δ13C values of N. pachyderma (s.) are basically invariant with depth in this region, are consistently 1.0‰ depleted in comparison with the δ13C for equilibrium calcite, and remain basically constant during the shell‐thickening process. Mass balance calculations suggest that encrustation occurs at all depths, but abundance counts suggest that the process occurs mostly at the depth of the main pycnocline. Sediment fluxes of N. pachyderma (s.) occur during a 2‐week bloom event and decrease to almost zero below complete ice cover. The decoupling of the processes controlling abundances and shell chemistry explain the discrepancies between transfer function and isotopically derived paleotemperature estimates of surface conditions, in some oceanic settings. The ability of δ18O to record surface ocean conditions will depend on vertical water column gradients, as evidenced by the differences in core‐top calibrations between the North and South Atlantic.