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Since the 1980–1990s, international research efforts have augmented our knowledge of the physical and chemical properties of the Arctic Ocean water masses, and recent studies have documented changes. Understanding the processes responsible for these changes is necessary to be able to forecast the local and global consequences of these property evolutions on climate. The present work investigates the distributions of geochemical tracers of particle fluxes and circulation in the Amerasian Basin and their temporal evolution over the last three decades (from stations visited between 1983 and 2015). Profiles of 230‐thorium (230Th) and 231‐protactinium (231Pa) concentrations and neodymium isotopes (expressed as εNd) measured in the Amerasian Basin prior to 2000 are compared to a new, post‐2000s data set. The comparison shows a large scale decrease in dissolved 230Th and 231Pa concentrations, suggesting intensification of scavenging by particle flux, especially in coastal areas. Higher productivity and sediment resuspension from the shelves appear responsible for the concentration decrease along the margins. In the basin interior, increased lateral exchanges with the boundary circulation also contribute to the decrease in concentration. This study illustrates how dissolved 230Th and 231Pa, with εNd support, can provide unique insights not only into changes in particle flux but also into the evolution of ocean circulation and mixing., Key Points Concentration decreases of 230Thd and 231Pad suggest an enhancement of particulate scavenging since the 2000s in the Amerasian BasinParticulate scavenging results from higher productivity and sediment resuspension from the shelvesPost‐2000s changes in intermediate and deep layers suggest enhanced lateral mixing between the margins and central areas of the basins

A high-resolution dinoflagellate cyst analysis on a sediment core GLW31D from the northern South China Sea (SCS) was performed to reconstruct paleoceanographic conditions over the last 12,500 years through qualitative, semi-quantitative, and quantitative methods. A modern dataset with 398 reference sites in the northern Pacific was assembled and used to identify the relationship between dinoflagellate cyst assemblages and sea-surface temperature (SST), sea-surface salinity (SSS) and primary productivity (PP). Modern analog technique (MAT) was applied to offer first dinoflagellate-cyst-based quantitative estimates of Holocene sea-surface conditions in the western North Pacific. The downcore reconstructions show that SST, SSS and PP were predominantly controlled by the changes in coastal and oceanic currents due to the changes in sea level and monsoon systems. Our results indicate that SST increased while SSS and PP decreased from 12,500 to ~6800 cal yr BP, reaching the maximum SST and the minimum SSS and PP during ~6800–5000 cal yr BP, and followed by a slight decline in SST with minor increases in SSS and PP. The three intervals correspond to the regional onshore sea-level stages of rising, stabilization in a highstand and slight drop, respectively. The Kuroshio Current strongly influenced the core site before ~9900 cal yr BP, reflected by the highest abundances of oceanic Impagidinium spp. and high reconstructed SSS values. This can be explained by a lack of water input from the East China Sea before the opening of the Taiwan Strait. The warmest period, from ~6800 cal yr BP to ~5500 cal yr BP, is recorded by the highest Dapsilidinium pastielsii abundances. Two short-term high-PP events of ~2700–2400 cal yr BP and ~1000–600 cal yr BP, which were characterized by opposite climatic conditions, coincided with two notable societal (dynasty) collapses of China. Enhanced anthropogenic activities since the Late Bronze Age most likely partially affected the high PP through influencing river inputs to the northern SCS.

The neodymium isotope composition ( ɛ Nd) of authigenic phases in marine sediment is widely used to reconstruct the origin and mixing of water masses of overlying seawater through time. However, at some locations in the modern ocean, the ɛ Nd of authigenic phases in surface sediment is not consistent with that of local seawater, raising concerns about its current interpretation as a paleotracer of water masses. To further investigate this question, we conducted a laboratory-based incubation experiment with a Mn-oxide phase placed at the sediment–water interface of multicores to assess the extent to which the authigenic phase records seawater ɛ Nd. Multicores were collected from the Strait of Georgia (SoG), which is a relatively deep coastal waterway with high sedimentation rates, oxygenated surface sediments, and active macrofauna, separating the mainland coast of British Columbia and Vancouver Island. Manganese oxide-coated XAD resin beads were placed at the sediment surface and the cores were incubated for 6 months in a tank filled with SoG seawater spiked with 150 Nd. While the ɛ Nd of the Mn-oxide coated resin (−4.0) was similar to that of SoG seawater used for the incubation (−3.7), the 150 Nd/ 144 Nd of the Mn-oxide phase measured after the incubation indicates that, under our experimental conditions, a minimum of 83% of the Nd associated with the Mn-oxide phase is not sourced from seawater, but from pore water. The 150 Nd/ 144 Nd ratio of the Mn-oxide resin is necessary to determine the predominant source of Nd to the resin because the ɛ Nd of SoG pore water (−3.9) is within analytical error of seawater (−3.7). Using field data and constraints from the Nd mass balance during the incubation, we conclude that the similarity of ɛ Nd in pore water and seawater in the SoG is fortuitous and not a result of a top-down or bottom-up control. Although the setting of our sediments is not directly comparable to open ocean locations, this study raises concerns about the use of ɛ Nd in paleocirculation studies, and points to the necessity of elucidating the factors controlling local lithogenic dissolution in pore waters as a prerequisite for the correct interpretation of ɛ Nd in the authigenic phases of marine sediments.

Chromium (Cr) stable isotopes have emerged as a powerful tool for tracking environmental redox transfor- mations. This is because Cr isotopes are fractionated during redox reactions between Cr(III) and Cr(VI). In order to fully exploit the information recorded within Cr isotope compositions, we must be able to track changes in Cr speciation throughout the environment and, in particular, the changes in speciation between input to the ocean and eventual deposition in sediments. We must also be able to access the isotope compositions of each Cr species, rather than only total dissolved Cr. We have thus developed a magnesium hydroxide coprecipitation method that meets these objectives. This method achieves complete recovery and has a typical precision on concentration measurements of !8% (1σ). It was tested using seawater collected from Saanich Inlet, a persis- tently anoxic fjord on the Pacific coast of Canada. Chromium speciation profiles and proof-of-concept isotope ratio measurements on selected samples indicate that isotopically lighter Cr(III) can be isolated from coexisting isotopically heavier Cr(VI), effectively resolving species-specific Cr isotope compositions. While the oxygenated surface waters of Saanich Inlet follow the generally observed correlation between seawater Cr concentration and its isotopic composition, seawater from anoxic depths diverges from this array, indicating that different pro- cesses are responsible for setting the isotope composition of these deeper waters. Broader application of Mg(OH)2 coprecipitation has strong potential to yield new insights into the fractionation of Cr isotopes in the oceans and the pathways that ultimately set the Cr isotopic composition of marine sediments and sedimentary archives.

Hydrothermal chromium (Cr) cycling contributes to marine Cr inventories and their Cr isotopic composition, yet Cr isotope effects associated with this cycling remain poorly documented. Here we determine the distribution, isotopic composition, and diagenetic mobility of Cr in hydrothermal sediments from the distal flank of the South East Pacific Rise (SEPR, DSDP-site 598). We find that Cr is primarily associated with the metalliferous iron (oxyhydr) oxide and detrital components of the sediment (0.4–3.6 mg kg⁻¹), whereas Cr concentrations are much lower in the dominant carbonate phase (80% Cr from the sediment relative to Fe. We propose this loss is tied to oxidation of authigenic Cr(III) to Cr(VI) followed by diagenetic remobilization and efflux from the sediment pile. The bulk δ⁵³Cr composition of the SEPR sediments is isotopically light (−0.24 to −0.57 ± 0.05‰) and the authigenic δ⁵³Cr is as light as −1.2 ± 0.2‰, and we argue that this light Cr isotopic composition results from the partial reduction of oxic seawater-bearing Cr(VI) by reduced hydrothermal vent fluids enriched in Fe(II)aq. Diagenetic oxidation of the reactive Cr pool by Mn-oxides and loss of Cr(VI) from the sediment may further deplete the sediment in ⁵³Cr during diagenesis. The δ⁵³Cr composition of the detrital Cr fraction of the sediment (average δ⁵³Cr composition = −0.05 ± 0.04‰) falls within the igneous silicate earth (ISE) range, revealing that detrital Cr delivered to this region of the Pacific ocean is unfractionated, and has carried a relatively constant δ⁵³Cr composition over the last 5.7 million years. Together our results show that light δ⁵³Cr compositions in hydrothermal sediments are imparted through a combination of processes previously overlooked in the marine Cr biogeochemical cycle, and that the δ⁵³Cr composition of such sediments may provide a rich source of information on paleo-marine redox conditions.

Trace elements of marine shells are important tools for reconstructing past ocean conditions, which are usually obtained through empirical calibrations with ocean parameters. For example, Mg/Ca ratios of ostracod shells have been linked to ocean temperature. However, some uncertainties usually arise from extraneous impacts, such as the selection of cleaning methods, specimen ontogenetic stage and shell preservation. Here, we present an evaluation of trace elements (Ca, Mg, Sr, Na, Al and Fe) in a shallow marine ostracod, Sinocytheridea impressa, and their potential utility as paleoceanographic proxies. We investigated single-shell variability of trace-element-to‑calcium ratios through Inductively Couple Plasma Mass Spectrometry (ICP-MS). In addition, the internal distribution of trace-elements was studied by Electron Probe Micro Analysis (EPMA) and Flow-Through Time-Resolved Analysis (FT-TRA). We conducted these analyses on ostracod specimens of different life stages and preservation conditions, subject to different cleaning methods, to resolve the suitability of this species as a new paleoenvironmental archive. Based on cleaning tests, we observe that clays stuck to the shells are the major contaminants in our specimens. We report an optimal cleaning procedure using sodium hypochlorite, which reduced the adherence of clays to the organism without significantly dissolving adult and juvenile calcite. We additionally find that the internal Mg distribution of S. impressa specimens is different for adults and the oldest stage of juveniles, namely A-1, with only the former having an enriched inner Mg layer. Therefore, precautions should be taken when combining life stages for trace-element analysis. While Sr and Mn are found to be mostly homogenous throughout all shells, the distribution of Na was found to be heterogeneous, which suggests that the incorporation of Na is controlled by a different mechanism than Mg, Sr and Mn. Finally, we report that transparent and opaque shells show similar Mg/Ca, Sr/Ca and Na/Ca ratios, unlike Mn/Ca and Fe/Ca, which suggests that opaque shells result from Mn- and Fe-enriched porewaters and Mn-enriched waters from the Pearl River. Our combined analysis suggests that Mg, Sr and Mn of adult S. impressa are incorporated into the carbonate matrix during calcification, and may therefore be suitable proxies of important environmental variables such as temperature and aqueous elemental concentrations.

Sedimentary 231Pa/230Th ratios have been used in the Atlantic Ocean as a proxy to reconstruct past changes in the strength and geometry of the Atlantic Meridional Overturning Circulation (AMOC), stemming from the southward export of 231Pa by the North Atlantic Deep Water (NADW). However, deep waters in the Pacific Ocean have a longer residence time, so that Pacific sedimentary 231Pa/230Th ratios are more strongly affected by particle/boundary scavenging, which obscures the influence of deep water circulation. The goal of this study is to examine how the Pacific Meridional Overturning Circulation (PMOC) affects the distribution of 231Pa and 230Th in the water column and sediment of the Pacific Ocean, and to explore whether sedimentary 231Pa/230Th ratios could provide useful constraints on past changes of the deep Pacific circulation. We examine the existing Pacific sedimentary 231Pa/230Th database and demonstrate that the Pacific Meridional Overturning Circulation (PMOC) has a distinctive influence on the distribution of sedimentary 231Pa/230Th. Using a 2D scavenging model with a simplified representation of the Pacific deep water circulation, we show that Pacific sedimentary 231Pa/230Th could potentially provide information on past variability in PMOC.

We examine the dissolved inorganic carbon maximum in the Canada Basin halocline using a suite of geochemical tracers to gain insight into the factors that contribute to the persistence of this feature. Hydrographic and geochemical samples were collected in the upper 500 m of the southwestern Canada Basin water column in the summer of 2008 and fall of 2009. These observations were used to identify conservative and nonconservative processes that contribute dissolved inorganic carbon to halocline source waters, including shelf sediment organic matter remineralization, air-sea gas exchange, and sea-ice brine export. Our results indicate that the remineralization of organic matter that occurs along the Bering and Chukchi Sea shelves is the overwhelming contributor of dissolved inorganic carbon to Pacific Winter Water that occupies the middle halocline in the southwestern Canada Basin. Nonconservative contributions from air-sea exchange and sea-ice brine are not significant. The broad salinity range associated with the DIC maximum, compared to the narrow salinity range of the nutrient maximum, is due to mixing between Pacific and Atlantic water and not abiotic addition of DIC.

Uranium isotope fractionation was studied in the sediment and water of Saanich Inlet, a seasonally anoxic fjord on the east coast of Vancouver Island. The concentration of dissolved U is the same above and below the redoxcline at ∼120 m depth, with an average δ238U value of −0.45 ± 0.06‰ (2 s.e.), which is indistinguishable from seawater values reported in other studies. This finding is consistent with water renewal times for the inlet that are roughly seasonal in frequency, thus hiding the impact of the U losses to the anoxic sediment of the inlet. Complete digests of the anoxic sediment yielded higher δ238U values than seawater (−0.21 ± 0.11‰ (2 s.e.)). By contrast, complete digests of the sediment trap samples yielded lower δ238U values than seawater (−0.52 ± 0.10‰ (2 s.e.)), and marine plankton from the inlet yielded the lowest δ238U value of −1.24 ± 0.17‰ (2σ). Cross-plots of δ238U and δ234U vs. Th/U reveal strong correlations indicative of two-component mixing when the data from traps and anoxic inlet sediment are combined. One mixing end-member is fractionated uranium of seawater origin, with a δ238U value of 0.17 ± 0.14‰ (2σ). The other is detrital uranium with a δ238U value of −0.83 ± 0.12‰ (2σ). The detrital end-member is lower than the range of δ238U values reported in the literature for granitoid igneous rocks (−0.44‰ to –0.17‰, 2σ) (Telus et al., 2012), suggesting that continental weathering fractionates uranium isotopes, with preferential release of 238U. Development and application of U isotopes as a paleoredox proxy has its basis in the nuclear volume fractionation. The data from Saanich Inlet meets this expectation, with a positive fractionation factor ( Δ aq(VI) sed(IV) ) of 0.62 ± 0.17‰ (2σ) calculated as the difference in δ238U between authigenic U in anoxic sediments and sediment traps (+0.17‰) and U dissolved in seawater (−0.45‰). However, it is widely believed that U(VI) reduction in the marine environment occurs on the surfaces of particles and that the negative isotope effect associated with U(VI) sorption to plankton ( Δ seawater plankton = - 0.79 ± 0.17 ‰ (2σ) opens up the possibility that the particulate pool of seawater derived U(VI) is fractionated from the dissolved pool. Accordingly, the reduction of U(VI) in the marine environment might involve two steps: (1) U(VI) sorption to particles with a negative fractionation, and (2) reduction to U(IV) on particles with a positive fractionation. Environmental factors that are not yet well understood may influence the relative reaction rates for the two steps, thus affecting the magnitude and sign of the overall fractionation. This has the potential to explain conflicting results reported for U(VI) reduction experiments in the literature (laboratory and field), where both positive and negative fractionations have been observed, and in some instances, no fractionation at all. Variation in the fractionation factor has the potential to complicate application of U isotopes as a paleoredox proxy, but this study and a study of the Black Sea (Romaniello, 2012) give the same results (within their respective uncertainties) suggesting that 0.62 ± 0.17‰ (2σ) is a robust assessment of the apparent U isotope fractionation factor associated with reductive deposition of seawater U(VI) in both anoxic and euxinic marine sediments of the present day.

A 2-year time series sampling with moored sediment traps deployed in Saanich Inlet, British Columbia, provides a monthly record of particulate organic carbon (POC) flux at three depths (50 m, 115 m, 180 m), which is compared to concurrent POC flux estimates obtained from 234 Th: 238 U disequilibrium. The results obtained by both methods are in general agreement, supporting the validity of both approaches for measuring POC flux in these coastal waters. Closer comparison indicates that the 234 Th-derived fluxes at 50 m are lower than measured by sediment traps. This difference is attributed to particle focusing from the narrowing of the fjord with depth, which leads to overestimating fluxes with sediment traps while underestimating these same fluxes with 234 Th. Higher fluxes measured at greater depths and better agreement between the two methods are consistent with lateral input of sediment winnowed from the sill by tidal currents. The very high scavenging intensity at our study site results in large 234 Th deficits and low dissolved 234 Th activities. This limits lateral 234 Th transport and the system largely maintains steady state. Our results indicate that the sinking flux of POC could be reasonably estimated by simply measuring 234 Th and organic carbon in suspended particles at our study site. Such a relatively simple method may be applicable to other coastal settings with similar high scavenging intensity and limited exchange with the open ocean and would facilitate the implementation of sampling programs to monitor carbon cycling or contaminants dispersion in this type of environment.

Boundary scavenging, or the enhanced removal of adsorption-prone elements from the ocean in areas of high particle flux, is an often cited, though not well-quantified, concept used to understand the oceanic distribution of many trace metals. Because 230 Th and 231 Pa are produced uniformly from uranium decay and removed differentially by scavenging, the process of boundary scavenging can be elucidated by a more detailed knowledge of their water column distributions. To this end, filtered seawater was collected across the gradients in particle flux which span the subarctic Pacific: in the west during the Innovative North Pacific Experiment (INOPEX) and in the east along Line P. Lateral concentration gradients of dissolved 230 Th are small throughout the subarctic Pacific at 12 sites of variable particle flux. This contradicts the prediction of the traditional boundary scavenging model. A compilation of water column data from throughout the North Pacific reveals much larger lateral concentration gradients for 230 Th between the subarctic North Pacific and subtropical gyre, over lateral gradients in scavenging intensity similar to those found within the subarctic. This reflects a biogeochemical-province aspect to scavenging. Upper water column distributions of 231 Pa and 231 Pa/ 230 Th ratio are consistent with the influence of scavenging by biogenic opal, while deep waters (42.5 km) reveal an additional 231 Pa sink possibly related to manganese oxides produced at continental margins or ridge crests.

Trace elements serve important roles as regulators of ocean processes including marine ecosystem dynamics and carbon cycling. The role of iron, for instance, is well known as a limiting micronutrient in the surface ocean. Several other trace elements also play crucial roles in ecosystem function and their supply therefore controls the structure, and possibly the productivity, of marine ecosystems. Understanding the biogeochemical cycling of these micronutrients requires knowledge of their diverse sources and sinks, as well as their transport and chemical form in the ocean. Much of what is known about past ocean conditions, and therefore about the processes driving global climate change, is derived from trace-element and isotope patterns recorded in marine deposits. Reading the geochemical information archived in marine sediments informs us about past changes in fundamental ocean conditions such as temperature, salinity, pH, carbon chemistry, ocean circulation and biological productivity. These records provide our principal source of information about the ocean's role in past climate change. Understanding this role offers unique insights into the future consequences of global change. The cycle of many trace elements and isotopes has been significantly impacted by human activity. Some of these are harmful to the natural and human environment due to their toxicity and/or radioactivity. Understanding the processes that control the transport and fate of these contaminants is an important aspect of protecting the ocean environment. Such understanding requires accurate knowledge of the natural biogeochemical cycling of these elements so that changes due to human activity can be put in context. Despite the recognised importance of understanding the geochemical cycles of trace elements and isotopes, limited knowledge of their sources and sinks in the ocean and the rates and mechanisms governing their internal cycling, constrains their application to illuminating the problems outlined above. Marine geochemists are poised to make significant progress in trace-element biogeochemistry. Advances in clean sampling protocols and analytical techniques provide unprecedented capability for high-density sampling and measurement of a wide range of trace elements and isotopes which can be combined with new modelling strategies that have evolved from the World Ocean Circulation Experiment (WOCE) and Joint Global Ocean Flux Study (JGOFS) programmes. A major new international research programme, GEOTRACES, has now been developed as a result of community input to study the global marine biogeochemical cycles of trace elements and their isotopes. Here, we describe this programme and its rationale

Fall AGU Conference on Deciphering Isotope Signatures of Earth Surface and Critical Zone Processes, San Francisco, CA, DEC, 2014; International audience; The transport time of siliciclastic sediments from their continental sites of formation to their final location of deposition on the seafloor is an important parameter bearing on land-sea interactions, climate variability and understanding of marine sediment record. U-234/U-238 activity ratios of the lithogenic fraction from late Quaternary sediment deposited in the Okinawa Trough, East China Sea, were reported in this study. On basis of U-234/U-238 activity ratios, the comminution ages and transport times were calculated using recoil loss factors (f(alpha)) derived from different equations based on grain size distribution. The transport times were longer (approximately 200 +/- 100 kyr) for the Okinawa Trough sediments deposited between 27 and 14 ka, decreased gradually between 14 and 7 ka, and stayed relatively short (

Nineteen labs representing nine nations participated in the GEOTRACES intercalibration initiative that determined concentrations of 232Th, 230Th, 231Pa or 10Be in seawater, suspended particles or sediments. Results generally demonstrated good agreement among labs that analyzed marine sediments. Two sets of seawater samples, aliquots of particulate material filtered in situ, and/or aliquots of biogenic sediments were distributed to participating labs. Internal consistency among participating labs improved substantially between the first and second set of seawater samples. Contamination was a serious problem for 232Th. Standard Niskin™ bottles introduced no detectable contamination, whereas sample containers, reagents and labware were implicated as sources of contamination. No detectable differences in concentrations of dissolved 232Th, 230Th or 231Pa were observed among samples of seawater filtered through Nuclepore ™, Supor ™ or QMA™ (quartz) filters with pore diameters ranging between 0.4 and 1.0 μm. Isotope yield monitors equilibrate with dissolved Th in seawater on a time scale of much less than one day. Samples of filtered seawater acidified to a pH between 1.7 and 1.8 experienced no detectable loss of dissolved Th or Pa during storage for up to three years. The Bermuda Atlantic Time Series station will serve as a GEOTRACES baseline station for future intercalibration of 232Th and 230Th concentrations in seawater. Efforts to improve blanks and standard calibration are ongoing, as is the development of methods to determine concentrations of particulate nuclides, tests of different filtration methods, and an increasing awareness of the need to define protocols for reporting uncertainties.

Mineral dust plays an important role in the climate system by interacting with radiation, clouds, and biogeochemical cycles. In addition, natural archives show that the dust cycle experienced variability in the past in response to global and local climate change. The compilation of the DIRTMAP (Dust Indicators and Records from Terrestrial and MArine Palaeoenvironments) paleodust data sets in the last 2 decades provided a benchmark for paleoclimate models that include the dust cycle, following a time slice approach. We propose an innovative framework to organize a paleodust data set that builds on the positive experience of DIRTMAP and takes into account new scientific challenges by providing a concise and accessible data set of temporally resolved records of dust mass accumulation rates and particle grain size distributions. We consider data from ice cores, marine sediments, loess–paleosol sequences, lake sediments, and peat bogs for this compilation, with a temporal focus on the Holocene period. This global compilation allows the investigation of the potential, uncertainties, and confidence level of dust mass accumulation rate reconstructions and highlights the importance of dust particle size information for accurate and quantitative reconstructions of the dust cycle. After applying criteria that help to establish that the data considered represent changes in dust deposition, 45 paleodust records have been identified, with the highest density of dust deposition data occurring in the North Atlantic region. Although the temporal evolution of dust in the North Atlantic appears consistent across several cores and suggests that minimum dust fluxes are likely observed during the early to mid-Holocene period (6000–8000 years ago), the magnitude of dust fluxes in these observations is not fully consistent, suggesting that more work needs to be done to synthesize data sets for the Holocene. Based on the data compilation, we used the Community Earth System Model to estimate the mass balance of and variability in the global dust cycle during the Holocene, with dust loads ranging from 17.2 to 20.8 Tg between 2000 and 10 000 years ago and with a minimum in the early to mid-Holocene (6000–8000 years ago)., National Science Foundation (U.S.) (Grant 0932946), National Science Foundation (U.S.) (Grant 1003509), United States. Dept. of Energy (DOE-SC00006735)

A critical question regarding millennial-scale climate variability during the last glacial–interglacial transition is whether the ocean or the atmosphere triggered abrupt climate changes. Specifically, either reorganization of the ocean's thermohaline circulation, or changes in tropical atmosphere–ocean dynamics similar to the present-day El Nino Southern Oscillation pattern could be involved. A high-resolution reconstruction of sea-surface temperatures in the equatorial Pacific over the past 30,000 years points to the oceans as the trigger for change, at least during the events known as Heinrich event 1 and the Younger Dryas. Surface ocean conditions in the equatorial Pacific Ocean could hold the clue to whether millennial-scale global climate change during glacial times was initiated through tropical ocean–atmosphere feedbacks or by changes in the Atlantic thermohaline circulation1. North Atlantic cold periods during Heinrich events and millennial-scale cold events (stadials) have been linked with climatic changes in the tropical Atlantic Ocean and South America2,3,4, as well as the Indian and East Asian monsoon systems5,6, but not with tropical Pacific sea surface temperatures7. Here we present a high-resolution record of sea surface temperatures in the eastern tropical Pacific derived from alkenone unsaturation measurements. Our data show a temperature drop of ∼1 °C, synchronous (within dating uncertainties) with the shutdown of the Atlantic meridional overturning circulation during Heinrich event 1, and a smaller temperature drop of ∼0.5 °C synchronous with the smaller reduction in the overturning circulation during the Younger Dryas event. Both cold events coincide with maxima in surface ocean productivity as inferred from 230Th-normalized carbon burial fluxes, suggesting increased upwelling at the time. From the concurrence of equatorial Pacific cooling with the two North Atlantic cold periods during deglaciation, we conclude that these millennial-scale climate changes were probably driven by a reorganization of the oceans’ thermohaline circulation, although possibly amplified by tropical ocean–atmosphere interaction as suggested before8.

The oceanic cycle of cadmium is still poorly understood, despite its importance for phytoplankton growth and paleoceanographic applications. As for other elements that are biologically recycled, variations in isotopic composition may bring unique insights. This article presents (i) a protocol for the measurement of cadmium isotopic composition (Cd IC) in seawater and in phytoplankton cells; (ii) the first Cd IC data in seawater, from two full depth stations, in the northwest Pacific and the northwest Mediterranean Sea; (iii) the first Cd IC data in phytoplankton cells, cultured in vitro. The Cd IC variation range in seawater found at these stations is not greater than 1.5 eCd/amu units, only slightly larger than the mean uncertainty of measurement (0.8 eCd/amu). Nevertheless, systematic variations of the Cd IC and concentration in the upper 300 m of the northwest Pacific suggest the occurrence of Cd isotopic fractionation by phytoplankton uptake, with a fractionation factor of 1.6 ± 1.4 eCd/amu units. This result is supported by the culture experiment data suggesting that freshwater phytoplankton (Chlamydomonas reinhardtii and Chlorella sp.) preferentially take up light Cd isotopes, with a fractionation factor of 3.4 ± 1.4 eCd/amu units. Systematic variations of the Cd IC and hydrographic data between 300 and 700 m in the northwest Pacific have been tentatively attributed to the mixing of the mesothermal (temperature maximum) water (eCd/amu = −0.9 ± 0.8) with the North Pacific Intermediate Water (eCd/amu = 0.5 ± 0.8). In contrast, no significant Cd IC variation is found in the northwest Mediterranean Sea. This observation was attributed to the small surface Cd depletion by phytoplankton uptake and the similar Cd IC of the different water masses found at this site. Overall, these data suggest that (i) phytoplankton uptake fractionates Cd isotopic composition to a measurable degree (fractionation factors of 1.6 and 3.4 eCd/amu units, for the in situ and culture experiment data, respectively), (ii) an open ocean profile of Cd IC shows upper water column variations consistent with preferential uptake and regeneration of light Cd isotopes, and (iii) different water masses may have different Cd IC. This isotopic system could therefore provide information on phytoplankton Cd uptake and on water mass trajectories and mixing in some areas of the ocean. However, the very small Cd IC variations found in this study indicate that applications of Cd isotopic composition to reveal aspects of the present or past Cd oceanic cycle will be very challenging and may require further analytical improvements. Better precision could possibly be obtained with larger seawater samples, a better chemical separation of tin and a more accurate mass bias correction through the use of the double spiking technique.

Anthropogenically driven climate change will rapidly become Earth's dominant transformative influence in the coming decades. The oceanic biological pump—the complex suite of processes that results in the transfer of particulate and dissolved organic carbon from the surface to the deep ocean—constitutes the main mechanism for removing CO2 from the atmosphere and sequestering carbon at depth on submillennium time scales. Variations in the efficacy of the biological pump and the strength of the deep ocean carbon sink, which is larger than all other bioactive carbon reservoirs, regulate Earth's climate and have been implicated in past glacial-​interglacial cycles. The numerous biological, chemical, and physical processes involved in the biological pump are inextricably linked and heterogeneous over a wide range of spatial and temporal scales, and they influence virtually the entire ocean ecosystem. Thus, the functioning of the oceanic biological pump is not only relevant to the modulation of Earth's climate but also constitutes the basis for marine biodiversity and key food resources that support the human population. Our understanding of the biological pump is far from complete. Moreover, how the biological pump and the deep ocean carbon sink will respond to the rapid and ongoing anthropogenic changes to our planet—including warming, acidification, and deoxygenation of ocean waters—remains highly uncertain. To understand and quantify present-day and future changes in biological pump processes requires sustained global observations coupled with extensive modeling studies supported by international scientific coordination and funding.

The trapping efficiency of bottom-tethered deep-sea sediment traps deployed in four oceanic basins was estimated from the intercepted fluxes of and . The results validate the general use of baffled, conical sediment traps for measuring the settling flux of particles in the bathypelagic zone (depth >1200 m) of the open ocean where current velocity is generally low. At shallower depths, within the mesopelagic zone, trapping efficiency tends to be lower and more erratic, even in areas of low current velocity (

Key Points: Use of sedimentary nitrogen isotopes is examined; On average, sediment 15N/14N increases approx. 2 per mil during early burial; Isotopic alteration scales with water depth Abstract: Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15N/14N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle.

Records of biogenic opal export in the North Atlantic Ocean show pronounced maxima during each glacial termination over the past 550,000 years, consistent with a strong deglacial reduction in the formation of silicate-poor glacial North Atlantic intermediate water and a consequent increase in upward silicate transport. The Southern Ocean is thought to have an important role in driving glacial/interglacial changes in atmospheric carbon dioxide concentrations, but the mechanism responsible for carbon dioxide release upon deglaciation remains unclear. Based on a record of biogenic opal export that exhibits opal maxima during each glacial termination of the past 550,000 years, Nele Meckler et al. propose that deglacial reduction in glacial North Atlantic intermediate water led to downward mixing of warm, low-density surface water ultimately triggering Antarctic overturning and carbon dioxide release to the atmosphere. Growing evidence suggests that the low atmospheric CO2 concentration of the ice ages resulted from enhanced storage of CO2 in the ocean interior, largely as a result of changes in the Southern Ocean1. Early in the most recent deglaciation, a reduction in North Atlantic overturning circulation seems to have driven CO2 release from the Southern Ocean2,3,4,5, but the mechanism connecting the North Atlantic and the Southern Ocean remains unclear. Biogenic opal export in the low-latitude ocean relies on silicate from the underlying thermocline, the concentration of which is affected by the circulation of the ocean interior. Here we report a record of biogenic opal export from a coastal upwelling system off the coast of northwest Africa that shows pronounced opal maxima during each glacial termination over the past 550,000 years. These opal peaks are consistent with a strong deglacial reduction in the formation of silicate-poor glacial North Atlantic intermediate water2 (GNAIW). The loss of GNAIW allowed mixing with underlying silicate-rich deep water to increase the silicate supply to the surface ocean. An increase in westerly-wind-driven upwelling in the Southern Ocean in response to the North Atlantic change has been proposed to drive the deglacial rise in atmospheric CO2 (refs 3, 4). However, such a circulation change would have accelerated the formation of Antarctic intermediate water and sub-Antarctic mode water, which today have as little silicate as North Atlantic Deep Water and would have thus maintained low silicate concentrations in the Atlantic thermocline. The deglacial opal maxima reported here suggest an alternative mechanism for the deglacial CO2 release5,6. Just as the reduction in GNAIW led to upward silicate transport, it should also have allowed the downward mixing of warm, low-density surface water to reach into the deep ocean. The resulting decrease in the density of the deep Atlantic relative to the Southern Ocean surface promoted Antarctic overturning, which released CO2 to the atmosphere.

During the Last Glacial Maximum, the pattern of Atlantic Meridional Overturning Circulation was different from today. A combination of sediment chemistry and a scavenging model suggests that the glacial circulation was shallower and at least as vigorous as today. The strength and geometry of the Atlantic meridional overturning circulation is tightly coupled to climate on glacial–interglacial and millennial timescales1, but has proved difficult to reconstruct, particularly for the Last Glacial Maximum2. Today, the return flow from the northern North Atlantic to lower latitudes associated with the Atlantic meridional overturning circulation reaches down to approximately 4,000 m. In contrast, during the Last Glacial Maximum this return flow is thought to have occurred primarily at shallower depths. Measurements of sedimentary 231Pa/230Th have been used to reconstruct the strength of circulation in the North Atlantic Ocean3,4, but the effects of biogenic silica on 231Pa/230Th-based estimates remain controversial5. Here we use measurements of 231Pa/230Th ratios and biogenic silica in Holocene-aged Atlantic sediments and simulations with a two-dimensional scavenging model to demonstrate that the geometry and strength of the Atlantic meridional overturning circulation are the primary controls of 231Pa/230Th ratios in modern Atlantic sediments. For the glacial maximum, a simulation of Atlantic overturning with a shallow, but vigorous circulation and bulk water transport at around 2,000 m depth best matched observed glacial Atlantic 231Pa/230Th values. We estimate that the transport of intermediate water during the Last Glacial Maximum was at least as strong as deep water transport today.

In two contrasting regions of the ocean, the equatorial Pacific and the southern ocean, the δ15N of core top sediments were strongly related to [NO3−] in surface waters. With distance from the equator in the equatorial Pacific, δ15N increased from 7‰ to 16‰ as [NO3−] decreased from 8μM to < 0.1 μM. Going from 60° to 30° S in the SE Indian Ocean, core top δ15N increased from 5‰ to 11‰ as surface [NO3−] decreased from 25μM to < 0.1 μM. These results are strong evidence that sedimentary δ15N in these regions is recording the increasing isotopic enrichment of near-surface NO3− with its depletion by phytoplankton. In the case of the equatorial Pacific, δ15N values for sinking particles collected at 150 m matched well the core top sediment values, demonstrating little diagenetic alteration of the near-surface generated isotopic signal. These equatorial Pacific data sets have variations with near-surface [NO3−] consistent with Rayleigh fractionation kinetics for a fractionation factor (ϵu) of 2.5‰. This value is substantially lower than previously found for temperate or polar regions, perhaps as a result of differences in phytoplankton species assemblage or growth condition. In the southern ocean south of the polar front, comparison of δ15N values for opal-rich sediments south and sinking particles indicates an apparent +5‰ diagenetic enrichment relative to the surface-generated signal that requires further investigation. This exception aside, our observations show that the surface-water relationship of increasing δ15N with increasing NO3− depletion is generally transmitted to and preserved in the sediments, an important requirement for further development and application of this important paleoceanographic tool.

A two dimensional scavenging-circulation model is used to investigate the patterns of sediment 231Pa/230Th generated by the Atlantic Meridional Overturning Circulation (AMOC) and further advance the application of this proxy for ocean paleocirculation studies. The scavenging parameters and the geometry of the overturning circulation cell have been chosen so that the model generates meridional sections of dissolved 230Th and 231Pa consistent with published water column profiles and an additional 12 previously unpublished profiles measured in the North and Equatorial Atlantic. The processes that generate the meridional sections of dissolved and particulate 230Th, dissolved and particulate 231Pa, dissolved and particulate 231Pa/230Th, and sediment 231Pa/230Th are discussed in detail. The results indicate that the relationship between sediment 231Pa/230Th at any given site and the overturning circulation is very complex. They clearly show that constraining past changes in the strength and geometry of the AMOC requires an extensive data set and they suggest strategies to maximize information from a limited number of samples.

[1] Studies from the subtropical western and eastern Atlantic Ocean, using the 231Pa/230Th ratio as a kinematic proxy for deep water circulation, provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation (MOC) over the last deglaciation. In this study, we present a compilation of existing and new sedimentary 231Pa/230Th records from North Atlantic cores between 1710 and 4550 m water depth. Comparing sedimentary 231Pa/230Th from different depths provides new insights into the evolution of the geometry and rate of deep water formation in the North Atlantic during the last 20,000 years. The 231Pa/230Th ratio measured in upper Holocene sediments indicates slow water renewal above ∼2500 m and rapid flushing below, consistent with our understanding of modern circulation. In contrast, during the Last Glacial Maximum (LGM), Glacial North Atlantic Intermediate Water (GNAIW) drove a rapid overturning circulation to a depth of at least ∼3000 m depth. Below ∼4000 m, water renewal was much slower than today. At the onset of Heinrich event 1, transport by the overturning circulation declined at all depths. GNAIW shoaled above 3000 m and significantly weakened but did not totally shut down. During the Bolling-Allerod (BA) that followed, water renewal rates further decreased above 2000 m but increased below. Our results suggest for the first time that ocean circulation during that period was quite distinct from the modern circulation mode, with a comparatively higher renewal rate above 3000 m and a lower renewal rate below in a pattern similar to the LGM but less accentuated. MOC during the Younger Dryas appears very similar to BA down to 2000 m and slightly slower below.

Changes in the orgC/CaCO3 ratio in particles sinking from the surface to the deep ocean have the potential to alter the atmospheric pCO2 over the span of a glacial/interglacial cycle. Recent paleoceanographic and modern observational studies suggest that silica is a key factor in the global carbon biogeochemical cycle that can influence the flux ratio, especially at low latitudes, through “silicic acid leakage” [Brzezinski, M., Pride, C., Franck, M., Sigman, D., Sarmiento, J., Matsumoto, K., Gruber, N., Rau, R., Coale, K., 2002. A switch from Si(OH)4 to NO 3 - depletion in the glacial Southern Ocean. Geophysical Research Letters 29, 5]. To test this hypothesis, we reconstruct biogenic fluxes of CaCO3, orgC and Si for three equatorial Pacific cores. We find evidence that a floral shift from a SiO2-based community to a CaCO3-based occurred, starting in mid-marine isotope stage (MIS) 3 (24–59 cal. ka) and declining toward MIS 2 (19–24 cal. ka). This could reflect the connection of the Peru upwelling system to the subantarctic region, and we postulate that excess silica was transported from the subantarctic via the deep Equatorial Undercurrent to the eastern equatorial Pacific. In the eastern equatorial Pacific only, we document a significant decrease in rain ratio starting mid-MIS 3 toward MIS 2. This decrease is concomitant with a significant decrease in silica accumulation rates at the seabed. This pattern is not observed in the Pacific influenced by equatorial divergence and shallow upwelling, where all reconstructed fluxes (CaCO3, orgC, and opal) increase during MIS 2. We conclude that the overall calcium carbonate pump weakened in the EEP under Peru upwelling influence.

Accurately estimating the vertical flux of material reaching the seafloor from the overlying surface waters is essential for the paleoceanographic reconstruction of a wide variety of oceanic processes. Two approaches are currently being used. One consists of estimating mass accumulation rates (MAR) between dated horizons as the product of linear sedimentation rates, sediment dry bulk densities, and concentrations. One pitfall with this approach is that sediments can be redistributed on the seafloor by bottom currents, and their accumulation may not necessarily reflect the true vertical rain rate originating from the overlying water column. To address this problem, the method of 230Th normalization was developed [Bacon, 1984]. This method is based on the assumption that the rapid scavenging of 230Th produced in the water column by decay of dissolved uranium results in its flux to the seafloor always being close to its known rate of production. To the extent that this assumption is correct, scavenged 230Th can be used as a reference to estimate the settling flux of other sedimentary constituents and to correct for sediment redistribution on the seafloor [Henderson and Anderson, 2003; Francois et al., 2004].

A first study from the subtropical western Atlantic, using 231Pa/230Th ratios as a kinematic proxy for deep water circulation, provided compelling evidence for a strong link between climate and the rate of Meridional Overturning Circulation (MOC) over the last deglaciation. However, these results warrant confirmation from additional locations and water depths because the interpretation of the sedimentary 231Pa/230Th ratio in terms of circulation vigor can be biased by variations in particle flux and composition. We have measured 231Pa/230Th in a core from the Iberian margin, in the Northeastern Atlantic basin, and have compared these new results to the data from the western Atlantic basin. We find that the reduction in the circulation during H1 and YD and the subsequent increases first recognized in the sediment deposited on Bermuda Rise are also evident in the eastern basin, in a totally different sedimentary regime, confirming that sedimentary 231Pa/230Th ratios record basin-wide changes in deep water circulation. However, some differences between the eastern and western records are also recognized, providing preliminary evidence to differentiate between renewal rates in the two North Atlantic basins and between shallower and deeper overturning. Our results suggest the possible existence of two sources of Glacial North Atlantic Intermediate Deep Water (GNAIW), one in the south Labrador Sea and another west of Rockall Plateau. Both sources contributed to the meridional overturning but the two had different sensitivity to meltwater from the Laurentide and the Fennoscandian ice sheets during the deglaciation. These results indicate that additional information on the geometry and strength of the ventilation of the deep Atlantic can be obtained by contrasting the evolution of sediment 231Pa/230Th in different sections of the Atlantic Ocean.

[1] The (231Pa/230Th)xs,0 records obtained from two cores from the western (MD97-2138; 1°25′S, 146°24′E, 1900 m) and eastern (Ocean Drilling Program Leg 138 Site 849, 0°11.59′N, 110°31.18′W, 3851 m) equatorial Pacific display similar variability over the last 85,000 years, i.e., from isotopic stages 1 to 5a, with systematically higher values during the Holocene, isotopic stage 3, and isotopic stage 5a, and lower values, approaching the production rate ratio of the two isotopes (0.093), during the colder periods corresponding to isotopic stages 2 and 4. We have also measured the 230Th-normalized biogenic preserved and terrigenous fluxes, as well as major and trace elements concentrations, in both cores. The (231Pa/230Th)xs,0 results combined with the changes in preserved carbonate and opal fluxes at the eastern site indicate lower productivity in the eastern equatorial Pacific during glacial periods. The (231Pa/230Th)xs,0 variations in the western equatorial Pacific also seem to be controlled by productivity (carbonate and/or opal). The generally high (231Pa/230Th)xs,0 ratios (>0.093) of the profile could be due to opal and/or MnO2 in the sinking particles. The profiles of (231Pa/230Th)xs,0 and 230Th-normalized fluxes indicate a decrease in exported carbonate, and possibly opal, during isotopic stages 2 and 4 in MD97-2138. Using 230Th-normalized flux, we also show that sediments from the two cores were strongly affected by sediment redistribution by bottom currents suggesting a control of mass accumulation rates by sediment focusing variability.

[1] The eastern equatorial Pacific (EEP) is an important center of biological productivity, generating significant organic carbon and calcite fluxes to the deep ocean. We reconstructed paleocalcite flux for the past 30,000 years in four cores collected beneath the equatorial upwelling and the South Equatorial Current (SEC) by measuring ex230Th-normalized calcite accumulation rates corrected for dissolution with a newly developed proxy for “fraction of calcite preserved.” This method produced very similar results at the four sites and revealed that the export flux of calcite was 30–50% lower during the LGM compared to the Holocene. The internal consistency of these results supports our interpretation, which is also in agreement with emerging data indicating lower glacial productivity in the EEP, possibly as a result of lower nutrient supply from the southern ocean via the Equatorial Undercurrent. However, these findings contradict previous interpretations based on mass accumulation rates (MAR) of biogenic material in the sediment of the EEP, which have been taken as reflecting higher glacial productivity due to stronger wind-driven upwelling.

The Atlantic meridional overturning circulation is widely believed to affect climate. Changes in ocean circulation have been inferred from records of the deep water chemical composition derived from sedimentary nutrient proxies1, but their impact on climate is difficult to assess because such reconstructions provide insufficient constraints on the rate of overturning2. Here we report measurements of 231Pa/230Th, a kinematic proxy for the meridional overturning circulation, in a sediment core from the subtropical North Atlantic Ocean. We find that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500 yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago. Following these cold events, the 231Pa/230Th record indicates that rapid accelerations of the meridional overturning circulation were concurrent with the two strongest regional warming events during deglaciation. These results confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate changes.

There is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates. Notwithstanding its common incidence, this process (generally referred to as sediment focusing) is often difficult to recognize. If redistribution is near synchronous to deposition, the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked. Ignoring it, however, can lead to serious misinterpretations of sedimentary fluxes, particularly when past changes in export flux from the overlying water are inferred. In many instances, this problem can be resolved, at least for sediments deposited during the late Quaternary, by normalizing to the flux of 230Th scavenged from seawater, which is nearly constant and equivalent to the known rate of production of 230Th from the decay of dissolved 234U. We review the principle, advantages and limitations of this method. Notwithstanding its limitations, it is clear that 230Th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates.

We report results from time-series decay and sequential leaching experiments of laboratory cultured and coastal plankton to elucidate the mechanisms controlling barite formation in seawater. Batch-cultured diatoms (Stephanopyxis palmerina) and coccolithophorids (Emiliania huxleyi) were let to decay in the dark for 8–10 weeks, suspended in aerated seawater. The development of barite crystals was monitored by Scanning Electron Microscopy (SEM). A similar experiment was conducted with plankton collected during the spring-bloom in Vineyard Sound (MA). In addition to SEM, suspended particles were sequentially leached for Ba (distilled water rinse; 10% (v/v) HNO3 rinse at room temperature; 30% (v/v) HCl at 80°C overnight; 50% (v/v) HNO3 at 80°C overnight) immediately after collection, and after 10-week decay in seawater, in seawater poisoned with HgCl2, and in seawater spiked with 135Ba. Both experiments showed an increase in the number of barite crystals during decay. The spring-bloom plankton had initially a large pool of labile Ba, soluble in distilled water and cold dilute HNO3 that was lost from the plankton after 10-week decay in both axenic and nonaxenic conditions. In contrast, Ba in the decayed plankton samples was predominantly in forms extracted by hot HCl and hot HNO3 acids, which were attributed to presence of barite Ba and refractory organic Ba respectively. The increase in barite crystal counts under a Scanning Electron Microscope (SEM), the increase in HCl extractable Ba relative to organic carbon, and the loss of a large fraction of Ba during plankton decay suggest that living plankton consists of a relatively large pool of labile Ba, which is rapidly released during plankton decomposition and acts as the main source of Ba for barite formation in supersaturated microenvironments. Since mass balance indicates that only a small proportion (2 to 4%) of the labile-Ba pool is converted to barite, the availability of microenvironments that could locally concentrate Ba released by plankton decay seems to be the main limiting factor in barite precipitation.

Difficulties in determining the 230Th and 231Pa concentration of seawater have hindered rapid progress in the application of these unique natural tracers of particle scavenging and ocean circulation. In response, we have developed an ICP/MS analytical procedure combining a degree of sensitivity, precision and sample throughput that can facilitate the systematic measurement of basin-scale changes in 230Th and 231Pa seawater concentration, and provide important constraints on circulation and mixing rates in the deep ocean. Seawater samples are spiked with 229Th and 233Pa and equilibrated before pre-concentration using conventional methods of Fe oxyhydroxide co-precipitation and anion exchange. Isotopic ratios are measured using a Finnigan MAT Element magnetic sector Inductively Coupled Plasma mass spectrometer (ICP/MS) equipped with a desolvating micronebulizer. Measurements are done on 10–20 l seawater samples with an internal precision of ∼2% and a reproducibility of ∼5% (95% confidence intervals (CI)) in deep water. After correction for procedural blank, 232Th tailing, and 232Th1H interference, the detection limits are ∼3 fg for 230Th and ∼0.4 fg for 231Pa. Applied to 20 l volumes, these detection limits correspond to concentrations of 0.15 fg/kg for 230Th and 0.02 fg/kg for 231Pa, which are 5–15 times lower than typical concentrations in surface water. The capability of this method is illustrated by two seawater profiles from the Equatorial Atlantic region that show systematic variations in 230Th and 231Pa concentration consistent with patterns of deep water circulation.

Treatment of diatom microfossils from Southern Ocean sediments with hot perchloric acid leaves a "diatom-bound" N fraction which is 0-4 parts per thousand lower in delta(15)N than the bulk sediment, typically 3 parts per thousand lower in recent Antarctic diatom ooze. Results from Southern Ocean surface sediments indicate that early diagenetic changes in bulk sediment N content and delta(15)N are not reflected in diatom-bound N, suggesting that diatom-bound N is physically protected from early diagenesis by the microfossil matrix. A meridional transect of multicores from the Indian sector of the Southern Ocean shows a northward increase in the delta(15)N Of diatom-bound N, suggesting that diatom-bound delta(15)N, like bulk sedimentary delta(15)N, varies with nitrate utilization in the overlying surface waters. The delta(15)N of diatom-bound N is 3-4 parts per thousand higher in glacial age Antarctic sediments than in Holocene sediments, supporting the hypothesis, previously based on bulk sediment delta(15)N, that nitrate utilization in the surface Antarctic was higher during the last ice age. While there are important uncertainties, the inferred range of utilization changes could potentially explain the entire similar to 80 ppm amplitude of observed glacial/interglacial variations in atmospheric CO2.

In large areas of the world’s oceans, there is a relationship between the mass flux of particulate matter and the unsupported 231Pa/230Th (xs 231Pa/xs 230Th) activity ratio of recent sediments. This observation forms the basis for using the xs 231Pa/xs 230Th ratio as a proxy for past changes in export productivity. However, a simple relationship between xs 231Pa/xs 230Th ratio and particle flux requires that the water residence time in an ocean basin is far in excess of the scavenging residence time of 231Pa, and that the composition of sinking particles maintains a strong preference for the adsorption of 230Th over 231Pa with a constant 230Th/231Pa fractionation factor (F). The best correlation between 231Pa/ 230Th ratio and mass flux is found in the Pacific Ocean. In the Atlantic, the contrast xs xs’ in the xs 231Pa/xs 230Th ratios between open ocean (low flux regions) and ocean margins (high flux regions) is much less pronounced due to the shorter residence time of deep water, resulting in less effective boundary scavenging of 231Pa. In the Southern Ocean, south of the Polar Front, there is no more a simple relationship between xs 231Pa/xs 230Th and particle flux. This is a result of a southward decrease in F, probably reflecting the increased opal content of sinking particles. Opal does not fractionate 231Pa and 230Th significantly. This lack of fractionation results in high xs 231Pa/xs 230Th ratios in opal-dominated regions, even in areas of very low particle fluxes such as the Weddell Sea. The xs 231Pa/xs 230Th ratio can therefore only be used as a paleoproductivity proxy if, in the time interval of interest, changes in the basin ventilation rate and differential scavenging of both radionuclides due to changes in the chemical composition of particulate matter can be excluded.

Burial of large quantities of magnetite (Fe(II)Fe(III)2O4) in iron formations (IFs) likely contributed to the protracted oxidation of Earth's surface during the Precambrian Eons. Magnetite can form through a diversity of biological and abiotic pathways and its preservation in IFs may thus be variably interpreted as the result of some combination of these processes. Such interpretations give rise to divergent pictures of the Precambrian Earth system and models for its evolution through time. New knowledge on the contribution of specific magnetite formation pathways is, therefore, needed to accurately tether our conceptual and numerical models to the geologic record. To constrain pathways of magnetite formation under ferruginous conditions, we conducted geochemical and multi-method microspectroscopic analyses on particles obtained from the water columns and sediments of ferruginous lakes Matano and Towuti, in Indonesia. We find that biologically reactive Fe(III) mineral phases are reduced in the anoxic waters of both lakes, causing the formation of primary authigenic magnetite, directly in the water column. This water column magnetite often takes conspicuous framboidal forms, which given the link to microbial Fe(III) reduction, may provide a biological signature on early Earth and by extension, other planetary bodies. The consumption of more biologically reactive forms of Fe(III) and the resulting delivery of primary magnetite to underlying sediments promotes the burial of oxidized equivalents and implies that primary magnetite formation could have been a principal pathway of Fe delivery to IFs. Combined, the removal of Fe from Earth's surface through biologically induced magnetite formation and subsequent burial in IFs, suggests that seawater chemistry and the microbially mediated reactions that cause magnetite formation played key roles in Earth system evolution and in setting the pace for planetary oxidation through the Precambrian Eons.