Source Water Masses Research Articles

Non-local drivers of the summer hypoxia in the East China Sea off the Changjiang Estuary

The East China Sea (ECS) off the Changjiang (Yangtze River) Estuary, located around the near field of the Changjiang plume (CJP) is a hot spot where phytoplankton blooms in the surface water and hypoxias in the subsurface/bottom waters are frequently observed. Based on field observations conducted in summer 2009 and 2011, we examined non-local drivers associated with the initial dissolved oxygen (DO) levels that had significant impact on the development of summer hypoxias in the ECS off the Changjiang Estuary. The bottom water mass therein could be traced isopycnally at 24.2 < σθ < 25.2 back to the vicinity of the Luzon Strait, ∼1300 km upstream, where subsurface Kuroshio water (∼220 m deep with ∼190 μmol DO kg−1) mixed with the South China Sea subsurface water (∼120 m deep with ∼130 μmol DO kg−1). Owing to the difference in DO of these two source water masses, their mixing ratio ultimately determined the initial DO supply to the ECS bottom water that eventually reached the hypoxic zone. This water mass mixture was also subject to biogeochemical alteration during its travel (∼60 days) after it intruded into the ECS at the northeastern tip of Taiwan. Along the pathway of the intruded bottom-hugging water, we found systematic increases in nutrient concentrations and apparent oxygen utilization, or drawdown in DO following Redfield stoichiometry as a result of marine organic matter decomposition. These non-local factors exerted a synergistic control on the initial DO of CJP bottom water promoting hypoxia formation, although the residence time of the CJP bottom water was relatively short (∼11 days). We contend that such far field drivers should be taken into account in order to better predict the future scenarios of coastal hypoxias in the context of global warming.

Seasonal variations in dissolved organic matter composition using absorbance and fluorescence spectroscopy in the Dardanelles Straits – North Aegean Sea mixing zone

The Dardanelles Straits – North Aegean Sea mixing zone is the area where the less saline waters of Black Sea origin supply organic material to the oligotrophic Mediterranean Sea. The objective of this work was to assess the seasonal dynamics of dissolved organic matter (DOM) in this region based on the optical properties (absorbance and fluorescence). By combining excitation-emission fluorescence with parallel factor analysis (EEM-PARAFAC), four fluorescent components were identified corresponding to three humic – like components and one amino acid – like. The latter was dominant during all seasons. Chromophoric DOM (CDOM) and dissolved organic carbon (DOC) were found to be strongly coupled only in early spring when conservative conditions prevailed and the two water masses present (Black Sea Waters – BSW and Levantine Waters – LW) could be identified by their absorption coefficients (a300) and spectral slopes S275–295. In summer and autumn the relationships collapsed. During summer two features appear to dominate the dynamics of CDOM: i) photodegradation that acts as an important sink for both the absorbing DOM and the terrestrially derived fluorescent humic substances and ii) the release of marine humic like fluorescent substances from bacterial transformation of DOM. Autumn results revealed a source of fluorescent CDOM of high molecular weight, which was independent of water mass sources and related to particle and sedimentary processes. The removal of the amino acid-like fluorescence during autumn provided evidence that although DOC was found to accumulate under low inorganic nutrient conditions, dissolved organic nitrogenous compounds could serve as bacterial substrate.

North Atlantic Deep Water Production during the Last Glacial Maximum.

Changes in deep ocean ventilation are commonly invoked as the primary cause of lower glacial atmospheric CO2. The water mass structure of the glacial deep Atlantic Ocean and the mechanism by which it may have sequestered carbon remain elusive. Here we present neodymium isotope measurements from cores throughout the Atlantic that reveal glacial–interglacial changes in water mass distributions. These results demonstrate the sustained production of North Atlantic Deep Water under glacial conditions, indicating that southern-sourced waters were not as spatially extensive during the Last Glacial Maximum as previously believed. We demonstrate that the depleted glacial δ13C values in the deep Atlantic Ocean cannot be explained solely by water mass source changes. A greater amount of respired carbon, therefore, must have been stored in the abyssal Atlantic during the Last Glacial Maximum. We infer that this was achieved by a sluggish deep overturning cell, comprised of well-mixed northern- and southern-sourced waters.

Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica

AbstractThe deep waters of the Bransfield Strait receive considerable amounts of water from the Weddell Sea continental shelf. The restricted connections to the surrounding ocean and relatively easier access makes the Bransfield Strait an important proxy region for monitoring changes in the dense Weddell Sea shelf water masses, which are an important precursor of Antarctic Bottom Water (AABW). Long‐term hydrographic data from the period 1960s–2010s showed freshening and lightening of the deep water masses of the Bransfield Strait, which was likely caused by large freshwater inputs originating from the western shelf of the Weddell Sea. The rates of freshening and lightening were −0.0010 ± 0.0005 yr−1 and −0.0016 ± 0.0014 kg m−3 yr−1 for the central basin, respectively, and −0.0010 ± 0.0006 yr−1 and −0.0029 ± 0.0013 kg m−3 yr−1 for the eastern basin, respectively. The deep waters showed a high degree of interannual thermohaline variability, which appeared to be caused by changes in the proportions of source water mass mixing between the years. Statistically significant negative correlations between salinity/neutral density fields and the Southern Annular Mode (SAM) were observed (−0.56 and −0.62 for the central basin, respectively, and −0.58 and −0.68 for the eastern basin, respectively) between 1980 and 2014. During SAM positive phases, communication between the Weddell Sea and the Bransfield Strait is reduced, which leads to less saline and lighter water masses in the Bransfield Strait; however, the opposite trends are observed during SAM negative phases.

Neodymium isotopic composition and concentration in the western North Atlantic Ocean: Results from the GEOTRACES GA02 section

The neodymium (Nd) isotopic composition of seawater is commonly used as a proxy to study past changes in the thermohaline circulation. The modern database for such reconstructions is however poor and the understanding of the underlying processes is incomplete. Here we present new observational data for Nd isotopes and concentrations from twelve seawater depth profiles, which follow the flow path of North Atlantic Deep Water (NADW) from its formation region in the North Atlantic to the northern equatorial Atlantic. Samples were collected during two cruises constituting the northern part of the Dutch GEOTRACES transect GA02 in 2010. The results show that the different water masses in the subpolar North Atlantic Ocean, which ultimately constitute NADW, have the following Nd isotope characteristics: Upper Labrador Sea Water (ULSW), εNd=−14.2±0.3; Labrador Sea Water (LSW), εNd=−13.7±0.9; Northeast Atlantic Deep Water (NEADW), εNd=−12.5±0.6; Northwest Atlantic Bottom Water (NWABW), εNd=−11.8±1.4. In the subtropics, where these source water masses have mixed to form NADW, which is exported to the global ocean, upper-NADW is characterised by εNd values of −13.2±1.0 (2sd) and lower-NADW exhibits values of εNd=−12.4±0.4 (2sd). While both signatures overlap within error, the signature for lower-NADW is significantly more radiogenic than the traditionally used value for NADW (εNd=−13.5) due to the dominance of source waters from the Nordic Seas (NWABW and NEADW). Comparison between the concentration profiles and the corresponding Nd isotope profiles with other water mass properties such as salinity, silicate concentrations, neutral densities and chlorofluorocarbon (CFC) concentration provides novel insights into the geochemical cycle of Nd and reveals that different processes are necessary to account for the observed Nd characteristics in the subpolar and subtropical gyres and throughout the vertical water column. While our data set provides additional insights into the contribution of boundary exchange in areas of sediment resuspension, the results for open ocean seawater demonstrate, at an unprecedented level, the suitability of Nd isotopes to trace modern water masses in the strongly advecting western Atlantic Ocean.

Dense-water bottom currents in the Southern Adriatic Sea in spring 2012

In February 2012, a severe cold spell in the European region triggered a massive production of very dense water on the northern Mediterranean Sea shelves. The spreading phase of the newly formed dense water was extensively studied in the Adriatic Sea by means of 2 ship surveys and 5 moorings fully equipped to monitor the flow of the bottom layer. For the Adriatic Sea, opposite to the Gulf of Lions, the area of cascading is far from the source area and this implies substantial modifications, adjustments and dilution of the source water mass along its path, with a spreading phase lasting several months. Indeed all the moorings detected events, although weaker than in the preceding months, until June 2012. The surveys detected 2 branches of NAdDW on the shelf, the first branch not denser than 29.7kg/m3 and the second branch not denser than 29.5kg/m3. Despite the extremely dense water generated in the Northern Adriatic, during events of dense-water flow, moorings recorded temperatures generally between 12.5 and 13°C, seldom less. Temperatures along the shelf break also did not fall below 13°C at depths greater than 400m. Turbulent mixing, therefore, heavily modified the cascading plumes, which left the shelf with thicknesses between 10 and 30m. Mooring data in the lowermost 100mab suggest that the thickness of the cascading layer increased by several tens of meters downslope, as a consequence of entrainment. Detraining frictional layers as well as locations of active cascading were identified mostly by isolated casts, highlighting the submesoscale domain of the downsloping plumes. The use of LADCP data allowed identification of very energetic bottom flow (40–50cm/s in many locations), with otherwise little signature in tracers, not previously observed. The Bari Canyon System (BCS) was so far recognized as a hot spot for cascading in the Southern Adriatic. However, during the 2012 event, this is not the only preferred site for cascading. Significant dense flow was detected in other locations. The northernmost mooring site, closer to the inception of the cascading process, in particular showed active cascading and several dynamical differences from the BCS: denser water with thinner boundary layer, events organized in multiple pulses with sub-inertial periodicity and with very short duration (12h to 1 day) that is generally not seen in other locations.

Oceanographic connectivity between right whale critical habitats in Canada and its influence on whale abundance indices during 1987–2009

The Roseway and Grand Manan basins on the Canadian Atlantic coast are neighboring late-summer critical feeding habitats for endangered North Atlantic right whales. Although in late summer these habitats regularly contain thick aggregations of right whale food – the copepod Calanus spp. – right whales periodically abandon one or both habitats in the same year. The causes of abandonments, their relationship to food supply, and the locations of whales during abandonment periods are unclear. The goals of this study were to explain variation in right whale abundance indices from a habitat perspective, and to determine whether or not oceanographic variation in the habitats influences occupancy. Four indices of whale abundance and habitat occupancy, including sightings per unit effort (SPUE), photographic sightings of known individuals, population size and habitat transition probabilities, were analyzed in relation to unique datasets of Calanus concentration and water mass characteristics in each basin over the period 1987 through 2009. Calanus concentration, water mass sources and various hydrographic properties each varied coherently between basins. Calanus concentration showed an increasing trend over time in each habitat, although a short-lived reduction in Calanus may have caused right whales to abandon Roseway Basin during the mid-1990s. Food supply explained variation in right whale sightings and population size in Roseway Basin, but not in Grand Manan Basin, suggesting that the Grand Manan Basin has important habitat characteristics in addition to food supply. Changes in the distribution of whale abundance indices during years when oceanographic conditions were associated with reduced food supply in the Scotia-Fundy region suggest that other suitable feeding habitats may not have existed during such years and resulted in negative effects on whale health and reproduction.

Global variability of chromium isotopes in seawater demonstrated by Pacific, Atlantic, and Arctic Ocean samples

Seawater chromium (Cr) isotope and concentration data are presented from multiple sites in the Arctic Ocean, and three locations in the Atlantic and Pacific Oceans. A 2400-m profile illustrates the heterogeneity of δ53Cr in the Arctic Ocean with depth and water-mass source (Pacific vs. Atlantic). The highest δ53Cr values occur in Pacific-sourced waters, which also have the lowest Cr concentration. Chromium concentration and δ53Cr data from these locations, in conjunction with published data for the South Atlantic Ocean, yield a simple logarithmic function, as predicted by Rayleigh fractionation in a closed system. The observed Cr isotope signature is hypothesized to arise from fractionation during the reduction of Cr(VI) in surface waters and oxygen minimum zones, scavenging of isotopically light Cr(III) to deeper water and sediment, and subsequent release of this seawater-derived Cr(III) back into seawater, either as organic complexes with Cr(III) or after oxidation to Cr(VI). The isotopic fractionation factor (ε) associated with Cr cycling in seawater is estimated to be −0.80±0.03‰ (2σ). Samples from the sea-ice affected Surface Mixed Layer of the Arctic Ocean (∼10 m depth) deviate from the general trend, and samples proximal to rivers illustrate geographic variation in δ53Cr values for continental runoff, but prompt loss of this signature away from the source.

Neodymium isotopes and concentrations in Caribbean seawater: Tracing water mass mixing and continental input in a semi-enclosed ocean basin

We present the first full water column Nd isotope (εNd) and concentration data for Caribbean seawater, as well as for stations close to the Orinoco River mouth and in the Florida Straits. The surface inflow into the southeastern Caribbean via the Guyana Current is characterized by an εNd signature of −10.9, which is a consequence of the mixing of relatively unradiogenic εNd signatures (−13.6) supplied by the Orinoco River with contributions from the Amazon River (∼−10). Despite the proximity to land, sub-surface and intermediate waters within the Caribbean largely retain the εNd signatures of their source water masses in the Atlantic. In contrast, the deep waters of the Caribbean show εNd signatures at least 3 εNd units more radiogenic than the inflowing Upper North Atlantic Deep Water (UNADW). A εNd shift from −13 to −9.7 can be explained by addition of radiogenic Nd to the deep Caribbean through weathering inputs from land. However, in order to balance such large shifts in εNd with at the same time modest increases in Nd concentrations, Nd must also be removed from seawater within the basin. It is suggested that the long residence time of deep waters in the Caribbean allows significant interaction of seawater with sinking particles and seafloor sediments resulting in more radiogenic values. These findings have implications for the use of εNd as a proxy for paleocirculation in restricted basins, in which the residence times of the deep waters are long.

Local and regional trends in Plio‐Pleistocene δ18O records from benthic foraminifera

AbstractWe present new orbital‐resolution Pliocene‐Pleistocene benthic stable oxygen isotope (δ18Ob) records from Ocean Drilling Program Sites 1264 and 1267, from Walvis Ridge in the Southeast Atlantic. We compare long‐term (&gt;250 kyr) interbasin δ18Ob‐gradients between Pacific and North Atlantic regional stacks, as well as intra and interbasin gradients from the perspective of Walvis Ridge. The δ18Ob values from Sites 1264 and 1267 are almost always higher than deep North Atlantic and Pacific sites, with large gradients (&gt;0.5‰) emerging abruptly at ∼2.4 Ma and persisting until ∼1.3 Ma. From this, we infer the presence of a new water mass, which resulted from the influence of dense, 18O‐enriched Nordic sea overflow waters via the abyssal East Atlantic. Meanwhile, long‐term average δ18Ob values in the North Atlantic appear to have remained within 0–0.25‰ lower than in the Pacific. However, the magnitude of this difference is sensitive to the inclusion of records from the equatorial West Atlantic. These results, together with constraints based on temperature, salinity, and density, suggest an influence of the seawater δ18O (δ18OSW) versus salinity relationship of source waters on δ18Ob values within the Atlantic. In particular, the abrupt emergence at ∼2.4 Ma of higher δ18Ob values at Sites 1264 and 1267, relative to North Atlantic records, appears to require a low‐latitude surface water δ18OSW signal. This implies a connection between northward heat transport and deep water export into the abyssal East Atlantic. Hence, our results have implications for the interpretation of δ18Ob records and highlight the potential for δ18Ob to constrain deep Atlantic water mass sources and pathways during the Plio‐Pleistocene.

A Fulcrum Approach To Assessing Source-To-Sink Mass Balance Using Channel Paleohydrologic Paramaters Derivable From Common Fluvial Data Sets With An Example From the Cretaceous of Egypt

Abstract Source-to-sink interpretations of genetic equivalency between fluvial feeder systems and basin accumulations infer that the sediment and water mass collected and transferred by rivers from the catchment is in balance with the mass ultimately delivered to the depositional basin. This relationship has value for modeling basin-fill volumes, climate and climate change, ocean water mass, and other applications. Executing an estimate of this mass balance is challenging in modern systems but even more difficult in deep-time stratigraphic systems where the catchment is no longer active and critical variables from the source and sink system are either not preserved or preserved with large uncertainties. Available data sets from stratigraphic systems are likewise often limited to a few localized boreholes, scattered outcrop, and/or geophysical surveys. This paper offers a method, the fulcrum test, for estimating mass flux from the source area to the basin sink by calculation of channel paleohydrologic variables extractable from these common stratigraphic data sources. We use the Cenomanian channels of the Bahariya Formation, Egypt, as an example application in a stratigraphic system. The technique may provide greater accuracy in modern systems where more data are available and uncertainties are lower. Total mass passing through a cross section of all feeder channels over a period of time should match with both the total sediment delivered to the cross section from the source area and the total volume delivered through these channels to the basin. This cross section would constitute a fulcrum across which source and sink sediment and water mass should balance. Bankfull dimensions and representative bedload are measured and sampled from channel stories identified in outcrop and/or subsurface data over a fulcrum cross section within the basin. Flow transport equations are used to estimate bankfull discharge and sediment concentrations using established methods. These concentrations are projected over longer durations to estimate total channel mass through-flux over basin-fill time spans. These estimates can also be tested against other mass-flux methods such as known volumetric basin-fill accumulation rates and/or estimates of drainage-area denudation. In the example case, calculations of mass flux from Bahariya channels that feed the equivalent fluvial-to-marine basin show that these channels were capable of delivering at least three times the sediment actually preserved. Channels were small with average depths of 2.5 m and 0.1 m3/sec bankfull sediment discharges. The fulcrum test offers a first-order approximation of mass balance, but it remains a nascent method. Key parameters have large uncertainties, which currently limit accuracy to an order of magnitude. This uncertainty could be reduced to a factor of three in stratigraphic systems through improved constraints on channel width and development of better relationships between bankfull and mean annual discharge. Uncertainties can be lowered to a factor of two in extant systems where key variables (e.g., slope, etc.) can be measured instead of estimated. The method also retains intrinsic limitations. It derives a discharge for only a single representative “unit” channel, and the contingency of multiple channels must be detected through other geologic data and integrated independently. The method also does not account for mass extraction through deposition between the catchment and the fulcrum, though fulcrum tests in multiple cross sections longitudinally could potentially lend insight into this issue. Accuracy of the test also depends upon stratigraphic preservation as a valid “statistical sampling machine” for discharge processes, and upon unbiased sampling of discrete channel stories within this preserved sample population.

Neodymium isotopic composition of intermediate and deep waters in the glacial southwest Pacific

Neodymium (Nd) isotopes, tracers of deep water mass source and mixing, were measured on sedimentary planktic foraminifera with authigenic coatings from a depth-transect of cores (1400–4800 m) from Chatham Rise in the southwest Pacific, over the past 30 ka. We observe deglacial variations in the Nd isotopic composition, which showed an average glacial composition of εNd=−5.0 (1σ; ±0.3n=4) for cores sites below 3200 mbsl. No significant deglacial variation was observed in the Nd isotopic composition of intermediate depth waters (1400 mbsl), in contrast with benthic foraminifera δC13 data. The deglacial εNd shift of CDW in the southwest Pacific is consistent with changes observed in the deep South Atlantic and Equatorial Indian Ocean, but εNd values are offset by ∼1εNd-unit to more radiogenic values throughout the deglacial records, likely due to admixture of a Nd isotope signal which was modified in the Southern Ocean or Pacific, perhaps by boundary exchange. However, this modification did not overprint the deglacial Nd isotope change. The consistent deglacial evolution of εNd in the South Atlantic, Equatorial Indian and southwest Pacific CDW, is evidence for the connection of CDW during the glacial, and propagation of diminished North Atlantic Deep Water export to the glacial Southern Ocean. In contrast, spatial heterogeneities in the benthic foraminifera δC13 of CDW have been observed in the Atlantic, Indian and Pacific basins of the deep glacial Southern Ocean. The Nd isotope data implies a well-connected deep Southern Ocean, which transported waters from the Atlantic to the Indian and Pacific oceans, during the glacial. This suggests that basin-scale variability in the glacial δC13 composition of CDW was unrelated to circulation changes.

Changing ocean circulation and hydrothermal inputs during Ocean Anoxic Event 2 (Cenomanian–Turonian): Evidence from Nd-isotopes in the European shelf sea

Nd isotopes of fish debris collected from the English Chalk at Eastbourne (Sussex, UK) are used to reconstruct the history of ocean circulation in the NW European shelf sea during Oceanic Anoxic Event 2 (OAE 2, Cenomanian–Turonian). The Eastbourne εNd record exhibits a 1-unit negative excursion (decreasing from ∼−9 to ∼−10), immediately followed by a 3-unit positive excursion reaching ∼−7. The onset of the negative εNd excursion lags the global δ13C rise characteristic of OAE 2, suggesting stable patterns of ocean circulation in the NW European shelf sea at this time. Both negative and positive Nd-isotope excursions took place during a transient cooling episode within OAE 2. The negative εNd excursion is interpreted as due to a change in ocean circulation with northerly sourced water masses becoming the dominant bottom waters at Eastbourne. The positive excursion is best explained by the transport of radiogenic Nd derived from a volcanic source, possibly the High Arctic or Caribbean large igneous province (LIP). An input of volcanic Nd may reconcile the Eastbourne record with coeval εNd records on Demerara Rise in the western tropical Atlantic. The broad synchroneity of high εNd values (∼−7) registered at both sites suggests a possible period with efficient oceanic mixing between the tropical Atlantic and the NW European shelf sea during the cooling episode. The Eastbourne εNd record of OAE 2, together with coeval temperature reconstructions, provides evidence for the coincidence of changes in ocean circulation and transient climatic cooling, implying a tight coupling between the two phenomena during this interval.

Variations in the strength of the North Atlantic bottom water during Holocene

We report here on a multi-proxy study of the changes in the dynamics and the properties of bottom water mass in the subpolar North Altantic during Holocene. Magnetic properties coupled with sortable silt and benthic carbon isotopes are investigated for Holocene marine sedimentary sequences located in the Charlie–Gibbs fracture zone (53°N) and in central (57°N) and southern Gardar drift (59°N). All the cores are located at water depths bathed by the Iceland–Scotland Overflow Water (ISOW), mixed at the southernmost locality with southern sourced water masses. The long-term variations in measured proxies are fitted with similar polynomial curves. An early Holocene event characterized by a shutdown/shoaling of the bottom circulation at the deepest sites is most likely related to the main deglacial freshwater inputs. It is followed by a progressive strengthening/deepening of the overflow water which culminates around 6kyr, in coincidence with the Holocene thermal maximum. After 6kyr corresponding to a drastic hydrological reorganization in the North Atlantic, a general decline in the bottom flow strength is observed until about 2kyr B.P. when it reached its present day state. After detrending, several short periods of reduced bottom flow strength and sedimentary transport from the northern detrital sources are observed, with a periodicity of around 600yr with no clear relationship at this time scale between surface and deep ocean.

Radium-based estimates of cesium isotope transport and total direct ocean discharges from the Fukushima Nuclear Power Plant accident

Abstract. Radium has four naturally occurring isotopes that have proven useful in constraining water mass source, age, and mixing rates in the coastal and open ocean. In this study, we used radium isotopes to determine the fate and flux of runoff-derived cesium from the Fukushima Dai-ichi Nuclear Power Plant (FNPP). During a June 2011 cruise, the highest cesium (Cs) concentrations were found along the eastern shelf of northern Japan, from Fukushima south, to the edge of the Kuroshio Current, and in an eddy ~ 130 km from the FNPP site. Locations with the highest cesium also had some of the highest radium activities, suggesting much of the direct ocean discharges of Cs remained in the coastal zone 2–3 months after the accident. We used a short-lived Ra isotope (223Ra, t1/2 = 11.4 d) to derive an average water mass age (Tr) in the coastal zone of 32 days. To ground-truth the Ra age model, we conducted a direct, station-by-station comparison of water mass ages with a numerical oceanographic model and found them to be in excellent agreement (model avg. Tr = 27 days). From these independent Tr values and the inventory of Cs within the water column at the time of our cruise, we were able to calculate an offshore 134Cs flux of 3.9–4.6 × 1013 Bq d−1. Radium-228 (t1/2 = 5.75 yr) was used to derive a vertical eddy diffusivity (Kz) of 0.7 m2 d−1 (0.1 cm2 s−1); from this Kz and 134Cs inventory, we estimated a 134Cs flux across the pycnocline of 1.8 × 104 Bq d−1 for the same time period. On average, our results show that horizontal mixing loss of Cs from the coastal zone was ~ 109 greater than vertical exchange below the surface mixed layer. Finally, a mixing/dilution model that utilized our Ra-based and oceanographic model water mass ages produced a direct ocean discharge of 134Cs from the FNPP of 11–16 PBq at the time of the peak release in early April 2011. Our results can be used to calculate discharge of other water-soluble radionuclides that were released to the ocean directly from the Fukushima NPP.

Distributions and sea-to-air fluxes of chloroform, trichloroethylene, tetrachloroethylene, chlorodibromomethane and bromoform in the Yellow Sea and the East China Sea during spring

Halocarbons including chloroform (CHCl3), trichloroethylene (C2HCl3), tetrachloroethylene (C2Cl4), chlorodibromomethane (CHBr2Cl) and bromoform (CHBr3) were measured in the Yellow Sea (YS) and the East China Sea (ECS) during spring 2011. The influences of chlorophyll a, salinity and nutrients on the distributions of these gases were examined. Elevated levels of these gases in the coastal waters were attributed to anthropogenic inputs and biological release by phytoplankton. The vertical distributions of these gases in the water column were controlled by different source strengths and water masses. Using atmospheric concentrations measured in spring 2012 and seawater concentrations obtained from this study, the sea-to-air fluxes of these gases were estimated. Our results showed that the emissions of C2HCl3, C2Cl4, CHBr2Cl, and CHBr3 from the study area could account for 16.5%, 10.5%, 14.6%, and 3.5% of global oceanic emissions, respectively, indicating that the coastal shelf may contribute significantly to the global oceanic emissions of these gases.

Obliquity forcing of organic matter accumulation during Oceanic Anoxic Event 2

An analysis of orbitally influenced climate‐sensitive sedimentation is conducted across a meridional transect of the proto‐North Atlantic, to reconstruct the behavior of the carbon cycle and climate during Oceanic Anoxic Event 2 (OAE 2; ∼94 Ma). Midlatitude to near equatorial sites spanning deep‐ocean to shelf environments are evaluated using a new inverse method for the identification and calibration of orbital influence in ancient strata. The results yield consistent independent astrochronologies at each location—compatible with the recently published radioisotopic/astrochronologic time scale at the Cenomanian‐Turonian GSSP—and thus provide a unified high‐resolution temporal context for evaluation of the event. The proto‐North Atlantic astrochronologies document an amplification of obliquity power during OAE 2, indicating that obliquity was the dominant pacemaker of organic matter accumulation by the latter portion of the event. The strong obliquity signal suggests an influence of high‐latitude climate processes across midlatitude deep‐ocean to tropical shelf environments. Changes in oceanic circulation during OAE 2 represent a likely mechanism for the propagation of a high latitude signal, and proxy data from Demerara Rise reveal the development of a new intermediate water mass source synchronous with the record of obliquity amplification. We hypothesize that a dense high‐latitude water mass displaced a pre‐existing nutrient‐rich anoxic deep‐water layer in the proto‐North Atlantic (augmented by enhanced volcanism), driving an episode of high productivity that lasted 700–800 ka. The development and intensification of the new intermediate/deep water source was potentially driven by eustatic sea level rise, which created expansive seaways in mid to high latitudes.

Rare earth element association with foraminifera

Neodymium isotopes are becoming widely used as a palaeoceanographic tool for reconstructing the source and flow direction of water masses. A new method using planktonic foraminifera which have not been chemically cleaned has proven to be a promising means of avoiding contamination of the deep ocean palaeoceanographic signal by detrital material. However, the exact mechanism by which the Nd isotope signal from bottom waters becomes associated with planktonic foraminifera, the spatial distribution of rare earth element (REE) concentrations within the shell, and the possible mobility of REE ions during changing redox conditions, have not been fully investigated. Here we present REE concentration and Nd isotope data from mixed species of planktonic foraminifera taken from plankton tows, sediment traps and a sediment core from the NW Atlantic. We used multiple geochemical techniques to evaluate how, where and when REEs become associated with planktonic foraminifera as they settle through the water column, reside at the surface and are buried in the sediment.Analyses of foraminifera shells from plankton tows and sediment traps between 200 and 2938m water depth indicate that only ∼20% of their associated Nd is biogenically incorporated into the calcite structure. The remaining 80% is associated with authigenic metal oxides and organic matter, which form in the water column, and remain extraneous to the carbonate structure. Remineralisation of these organic and authigenic phases releases ions back into solution and creates new binding sites, allowing the Nd isotope ratio to undergo partial equilibration with the ambient seawater, as the foraminifera fall through the water column.Analyses of fossil foraminifera shells from sediment cores show that their REE concentrations increase by up to 10-fold at the sediment–water interface, and acquire an isotopic signature of bottom water. Adsorption and complexation of REE3+ ions between the inner layers of calcite contributes significantly to elevated REE concentrations in foraminifera. The most likely source of REE ions at this stage of enrichment is from bottom waters and from the remineralisation of oxide phases which are in chemical equilibrium with the bottom waters.As planktonic foraminifera are buried below the sediment–water interface redox-sensitive ion concentrations are adjusted within the shells depending on the pore-water oxygen concentration. The concentration of ions which are passively redox sensitive, such as REE3+ ions, is also controlled to some extent by this process. We infer that (a) the Nd isotope signature of bottom water is preserved in planktonic foraminifera and (b) that it relies on the limited mobility of particle reactive REE3+ ions, aided in some environments by micron-scale precipitation of MnCO3.This study indicates that there may be sedimentary environments under which the bottom water Nd isotope signature is not preserved by planktonic foraminifera. Tests to validate other core sites must be carried out before downcore records can be used to interpret palaeoceanographic changes.

A boundary exchange influence on deglacial neodymium isotope records from the deep western Indian Ocean

The use of neodymium (Nd) isotopes to reconstruct past water mass mixing relies upon the quasi-conservative behaviour of this tracer, whereas recent studies in the modern oceans have suggested that boundary exchange, involving the addition of Nd from ocean margin sediments, may be an important process in the Nd cycle. Here we suggest that the relative importance of water mass advection versus boundary exchange can be assessed where the deep western boundary current in the Indian Ocean flows past the Madagascan continental margin; a potential source of highly unradiogenic Nd. Foraminiferal coatings and bulk sediment reductive leachates are used to reconstruct bottom water Nd isotopic composition (εNd) in 8 Holocene age coretops, with excellent agreement between the two methods. These data record spatial variability of ∼4 εNd units along the flow path of Circumpolar Deep Water; εNd≈−8.8 in the deep southern inflow upstream of Madagascar, which evolves towards εNd≈−11.5 offshore northern Madagascar, whereas εNd≈−7.3 where deep water re-circulates in the eastern Mascarene Basin. This variability is attributed to boundary exchange and, together with measurements of detrital sediment εNd, an isotope mass balance suggests a deep water residence time for Nd of ≤400yr along the Madagascan margin. Considering deglacial changes, a core in the deep inflow upstream of Madagascar records εNd changes that agree with previous reconstructions of the Circumpolar Deep Water composition in the Southern Ocean, consistent with a control by water mass advection and perhaps indicating a longer residence time for Nd in the open ocean away from local sediment inputs. In contrast, sites along the Madagascan margin record offset εNd values and reduced glacial–interglacial variability, underlining the importance of detecting boundary exchange before inferring water mass source changes from Nd isotope records. The extent of Madagascan boundary exchange appears to be unchanged between the Holocene and Late Glacial periods, while a consistent shift towards more radiogenic εNd values at all sites in the Late Glacial compared to the Holocene may represent a muted signal of a change in water mass source or composition.

Declining silicate concentrations in the Norwegian and Barents Seas

Abstract Rey, F. 2012. Declining silicate concentrations in the Norwegian and Barents Seas. – ICES Journal of Marine Science, 69: 208–212. Since 1990, a decline in silicate concentrations together with increasing salinities has been observed in the Atlantic water of the Norwegian and Barents Seas. This decline in silicate has been found to be related to the relative proportion in which eastern and western source water masses from the northeastern North Atlantic enter the Norwegian Sea.