Signatures of different water masses in δ<Superscript>18</Superscript>O values,relative abundance of <Emphasis Type="Italic">G. Bulloides</Emphasis> and total planktonic foraminiferal assemblage across the Indian Ocean sector of the southern ocean: Initial results

Nineteen surface sediment samples, collected onboard ORV Sagar Kanya during the 199C and 200th (the pilot expedition to Southern Ocean) cruises along a north-south transect between 1.92° S to 55.01° S latitude and 67.88° E and 44.89° E longitude, were studied for total planktonic foraminiferal assemblage, relative abundance and the oxygen isotopic values of the indicator planktic species Globigerina bulloides. These profiles were compared with the latitudinal variations in the average salinity and nutrient (phosphate) contents of the top 100 meters water column. The initial results have traced the signatures of salinity linked variations of different water masses in the study region. The influence of Australian Mediterranean Water (AAMW) has been noticed in the present dataset.     

Spatiotemporal variations of the δ18O–salinity relation in the northern Indian Ocean

A new data set of oxygen isotopic composition (δ¹⁸O) and salinity (S) of surface and sub-surface waters of the northern Indian Ocean, collected during the period 1987–2009, is presented. While the results are consistent with positive P?E (excess of precipitation over evaporation) over the Bay of Bengal and negative P?E over the eastern Arabian Sea, a significant spatiotemporal variability in the slope (also intercept) of the δ¹⁸O–S relation is observed in the Bay; the temporal variability is difficult to discern in the Arabian Sea. The slope and intercept are positively and negatively correlated, respectively, with the annual rainfall over India, a rough measure of river runoff. Both the slope and intercept appear to be sensitive to rainfall; the slope (intercept) is higher (lower) during years of stronger monsoon. The observed variability in the δ¹⁸O–S relation implies that caution needs to be exercised in paleosalinity estimations, especially from the Bay of Bengal, based on δ¹⁸O of marine organisms.     

Upwelling and productivity along the Peru–Chile Current derived from faunal and isotopic compositions of planktic foraminifera in surface sediments

We report on the spatial distribution of isotopic compositions of the two planktic foraminifera species Globigerina bulloides and Neogloboquadrina pachyderma (dex.), and the faunal assemblages of planktic foraminifera in 91 surface sediment samples along the Chilean continental slope between 23°S and 44°S. Both δ¹³C and δ¹⁸O data of N. pachyderma (dex.) show little variability in the study area. North of 39°S, the isotopic values of N. pachyderma (dex.) are heavier than those of G. bulloides, whereas south of 39°S, this relation inverses. This is indicative for a change from a well-mixed, deep thermocline caused by coastal upwelling north of 39°S to well-stratified water masses in a non-upwelling environment south of 39°S. In addition, the faunal composition of planktic foraminifera marks this change by transition from an upwelling assemblage north of 39°S to a high-nutrient-non-upwelling assemblage south of 39°S, which is characterized by decreased contributions of upwelling indicators such as G. bulloides, N. pachyderma (sin.), and Globigerinita glutinata. In general, we can conclude that food and light rather than temperature are the primary control of the planktic foraminiferal assemblage between 23°S and 44°S off Chile. Our data point to higher marine productivity at upwelling centers north of 25°S and at 30–33°S. South of 39°S, significant supply of nutrients by fluvial input most likely boosts the productivity.     

Do changes in coiling directions in planktonic foraminifera correspond to dimorphic reproduction?

A total of 25 surface sediment samples, collected along a North-South transect (from 9.69° N to 55.01° S and from 80° E and 40° E) in the south western Indian Ocean, were used to study the coiling direction patterns in foraminifera planktonic species Neogloboquadrina pachyderma, Globigerinita glutinata and Globigerina bulloides. Comparison between the coiling direction and mean proloculus size (MPS) revealed that all these profiles along N-S transect were not in tandem and thus indicated non-existence of any relationship between the coiling direction and reproductive modes expressed in terms of mean proloculus size.     

Depth habitats and seasonal distributions of recent planktic foraminifers in the Canary Islands region (29°N) based on oxygen isotopes

Seasonal depth stratified plankton tows, sediment traps and core tops taken from the same stations along a transect at 29°N off NW Africa are used to describe the seasonal succession, the depth habitats and the oxygen isotope ratios (δ¹⁸O_shell) of five planktic foraminiferal species. Both the δ¹⁸O_shell and shell concentration profiles show variations in seasonal depth habitats of individual species. None of the species maintain a specific habitat depth exclusively within the surface mixed layer (SML), within the thermocline, or beneath the thermocline. Globigerinoides ruber (white) and (pink) occur with moderate abundance throughout the year along the transect, with highest abundances in the winter and summer/fall season, respectively. The average δ¹⁸O_shell of G. ruber (w) from surface sediments is similar to the δ¹⁸O_shell values measured from the sediment-trap samples during winter. However, the δ¹⁸O_shell of G. ruber (w) underestimates sea surface temperature (SST) by 2 °C in winter and by 4 °C during summer/fall indicating an extension of the calcification/depth habitat into colder thermocline waters. Globigerinoides ruber (p) continues to calcify below the SML as well, particularly in summer/fall when the chlorophyll maximum is found within the thermocline. Its vertical distribution results in δ¹⁸O_shell values that underestimate SST by 2 °C. Shell fluxes of Globigerina bulloides are highest in summer/fall, where it lives and calcifies in association with the deep chlorophyll maximum found within the thermocline. Pulleniatina obliquiloculata and Globorotalia truncatulinoides, dwelling and calcifying a part of their lives in the winter SML, record winter thermocline (～180 m) and deep surface water (～350 m) temperatures, respectively. Our observations define the seasonal and vertical distribution of multiple species of foraminifera and the acquisition of their δ¹⁸O_shell.     

Isotope parameters (δD, δ<Superscript>18</Superscript>O) and sources of freshwater input to Kara Sea

The isotope characteristics (δD, δ¹⁸О) of Kara Sea water were studied for quantitative estimation of freshwater runoff at stations located along transect from Yamal Peninsula to Blagopoluchiya Bay (Novaya Zemlya). Freshwater samples were studied for glaciers (Rose, Serp i Molot) and for Yenisei and Ob estuaries. As a whole, δD and δ¹⁸O are higher in glaciers than in river waters. isotope composition of estuarial water from Ob River is δD =–131.4 and δ18O =–17.6‰. Estuarial waters of Yenisei River are characterized by compositions close to those of Ob River (–134.4 and–17.7‰), as well as by isotopically “heavier” compositions (–120.7 and–15.8‰). Waters from studied section of Kara Sea can be product of mixing of freshwater (δD =–119.4, δ¹⁸O =–15.5) and seawater (S = 34.9, δD = +1.56, δ¹⁸O = +0.25) with a composition close to that of Barents Sea water. isotope parameters of water vary significantly with salinity in surface layer, and Kara Sea waters are desalinated along entire studied transect due to river runoff. concentration of freshwater is 5–10% in main part of water column, and <5% at a depth of >100 m. maximum contribution of freshwater (>65%) was recorded in surface layer of central part of sea.     

Hydrography and circulation of the bay of Bengal during withdrawal phase of the southwest Monsoon

Hydrographic data were collected from 3 to 10 September 1996 along two transects; one at 18° N and the other at 90° E. The data were used to examine the thermohaline, circulation and chemical properties of the Bay of Bengal during the withdrawal phase of the southwest monsoon. The surface salinity exhibited wide spatial variability with values as low as 25.78 at 18° N / 87° E and as high as 34.79 at 8° N / 90° E. Two high salinity cells (S > 35.2) were noticed around 100 m depth along the 90° E transect. The wide scatter in T-S values between 100 and 200 m depth was attributed to the presence of the Arabian Sea High Salinity (ASHS) water mass. Though the warm and low salinity conditions at the sea surface were conducive to a rise in the sea surface topography at 18° N / 87° E, the dynamic height showed a reduction of 0.2 dyn.m. This fall was attributed to thermocline upwelling at this location. The geostrophic currents showed alternating flows across both the transects. Relatively stronger and mutually opposite currents were noticed around 25 m depth across the 18° N transect with velocity slightly in excess of 30 cm s⁻¹. Similar high velocity (> 40 cm s⁻¹) pockets were also noticed to extend up to 30 m depths in the southern region of the 90° E transect. However, the currents below 250 m were weak and in general < 5 cm s⁻¹. The net geostrophic volume transports were found to be of the order of 1.5 × 10⁶ m³ s⁻¹ towards the north and of 6 × 10⁶ m³ s⁻¹ towards west across the 18° N and 90° E transects respectively. The surface circulation patterns were also investigated using the trajectories of drifting buoys deployed in the eastern Indian Ocean around the same observation period. Poleward movement of the drifting buoy with the arrival of the Indian Monsoon Current (IMC) at about 12° N along the eastern rim of the Bay of Bengal has been noticed to occur around the beginning of October. The presence of an eddy off the southeast coast of India and the IMC along the southern periphery of the Bay of Bengal were also evident in the drifting buoy data.     

Latitudinal trends in morphological characteristics of <Emphasis Type="Italic">Neogloboquadrina pachyderma</Emphasis> (Ehrenberg) along a north–south transect in the south-western Indian Ocean

A total of 25 surficial sediment samples (Peterson grab, gravity and piston cores), collected during the Pilot Expedition to Southern Ocean (PESO) 2004 cruises 199C and 200 onboard the ORV Sagar Kanya along a N–S transect between 9.69°N and 55.01°S, and 80 and 40°E in the Indian Ocean sector of the Southern Ocean (SW Indian Ocean), have been investigated for various morphological features—test size, mean proloculus size and coiling direction (dextral/sinistral forms)—of the planktic indicator species Neogloboquadrina pachyderma (Ehrenberg). The results show that the coiling directions co-vary with temperature and salinity, the abundances of sinistrally coiled forms increasing towards higher latitudes (south of 40°S), whereas dextrally coiled forms show a reverse trend. Similarly, overall test and proloculus sizes depend largely on the physicochemical properties (salinity, temperature, nutrients, calcium saturation) of the ambient water masses. These observations suggest that, particularly at the boundaries between different water masses, variations in morphological features of N. pachyderma can meaningfully be used to reconstruct paleoceanographic conditions from Indian Ocean sediments.     

Carbon cycle and sea-water palaeotemperature evolution at the Middle–Late Jurassic transition,eastern Paris Basin (France)

A very high-resolution carbon and oxygen stable isotope analysis (bulk-carbonate) of a biostratigraphically well-constrained Callovian–Oxfordian series is provided here for the first time. The homogeneity of the clayey series and the weak diagenetic alteration allow the isotopic signal variations to be considered as primary in origin. A prominent and brief negative excursion in the δ¹³C curve (−2‰), occurring at the start of the Middle Callovian (Jason Zone – Obductum Subzone) and correlated regionally, suggests a possible methane release. The increasing δ¹³C values thereafter up to the Early Oxfordian, concomitant with a warming episode, highlight the burial of carbon in organic-rich layers which, in return, may have triggered a decrease in atmospheric pCO₂. At higher frequencies, observed fluctuations of the δ¹³C and δ¹⁸O values are orbitally driven (405-kyr and 100-kyr eccentricity cycles) and may correspond to the salinity and temperature variability recorded in sea water. The δ¹⁸O isotopic measurements from well-preserved diagenetically screened belemnites and bivalves along the series, compared to available data from Tethyan domains, agree with the scenario of a global cooling at the Middle-Late Jurassic transition. The well-dated δ¹⁸O isotopic curve suggests that the onset of this cooling event occurred at the end of the Coronatum Zone (Middle Callovian).     

Shell growth and oxygen isotopes in the topshell Osilinus turbinatus: resolving past inshore sea surface temperatures

Shells of the rocky shore intertidal gastropod Osilinus turbinatus (von Born), often abundant in archaeological deposits in the Mediterranean region, are a potential source of data on palaeotemperature, palaeoseasonality and archaeological seasonality. To evaluate this species as a climate archive, investigations of annual patterns of shell growth and of monthly variations in oxygen isotopes in shell carbonates were made on different populations in NW Sicily. Mark-recapture experiments at San Vito lo Capo and Mazzaforno show that O. turbinatus grows almost continuously throughout the year but at different rates in different seasons. Around 75% of the yearly shell growth occurs in the autumn and winter. On average, larger/older shells produce narrower annual growth increments than smaller/younger ones. Conspicuous growth lines in larger/older shells show that growth stops during the hottest part of the summer. Oxygen isotope analyses on monthly collected shells of O. turbinatus from three shores (Cala Grande, Monte Cofano and Mazzaforno) show that the isotope values record temperature variations through the year. In all the datasets, surface seawater temperatures (SSTs) calculated from δ¹⁸O_SHELL mostly underestimate measured SSTs, offsets being generally greater in summer. Minimum annual offsets range from 0.0°C to 0.7°C and maximum annual offsets from 3.1°C to 8.7°C. δ¹⁸O_SHELL values fail to record temperatures higher than 25°C. Careful selection of shells to be analysed can reduce offsets between δ¹⁸O_SHELL temperature estimates and measured SSTs for many parts of the year, except the hottest. Allowing for this, shells of O. turbinatus offer good potential as climate archives and for archaeological studies of seasonal patterns of human foraging for shellfish.     

Shells of Paphia undulata (Bivalvia) from the South China Sea as potential proxy archives of the East Asian summer monsoon: a sclerochronological calibration study

The climate of the South China Sea is dominated by the East Asian monsoon (EAM) system. Existing paleoclimate reconstructions offered an excellent insight into longer-term EAM variations. However, due to a lack of appropriate high-resolution paleoclimate data, relatively little is known about the frequency and strength of EAM extremes during the Holocene. To evaluate and establish a potential proxy archive for past variations of the EAM on shorter time-scales, we have carried out a calibration study on shells of the bivalve mollusk, Paphia undulata (Born 1778) from Daya Bay, China. This species has a short lifespan (3 years). Shells grow uninterruptedly between February/March and mid-November and are formed near oxygen isotopic (δ¹⁸O) equilibrium with the ambient environment. Shell growth patterns, δ¹⁸O_shell and δ¹³C_shell values, can be used to estimate the relative amount of precipitation and terrestrial runoff. Therefore, shells of this species can provide reliable, sub-seasonally resolved data on past East Asian summer monsoon strengths. The feasibility of this method has been tested with two Holocene shells from sediment cores taken from the nearby Beibu Gulf. A rather peculiar finding is that shell growth of P. undulata seems to be largely uncoupled to measured local environmental variables. Growth rates are negatively correlated to seawater temperature and chlorophyll a levels and positively to salinity. It is hypothesized here that extraordinary fast shell growth in early spring (February/March; low temperature and primary productivity) are facilitated by preserved energy resources, ensuring that the bivalve quickly reaches the predation window and the required size for reproduction.     

Oxygen isotopic distribution along the otolith growth axis by secondary ion mass spectrometry: Applications for studying ontogenetic change in the depth inhabited by deep-sea fishes

This study using tuna otoliths as working standards established a high lateral resolution and precision analysis to measure δ¹⁸O_otolith by secondary ion mass spectrometry. This analytical approach of the ion probe was applied to deep-sea fishes to reconstruct the likely depths inhabited by the fishes at different life history stages based on the measured δ¹⁸O_otolith values as a proxy of water temperature. Dramatic increases up to 5–6‰ in δ¹⁸O_otolith, representing a temperature decrease of approximately 20 °C, were detected in a blind cusk eel (Barathronus maculatus) otolith and in the otoliths of Synaphobranchus kaupii during leptocephalus metamorphosis to glass eel, inferred from the drop of otolith Sr/Ca ratios and increase of otolith growth increment width. δ¹⁸O_otolith profiles clearly divided the fish's life history into a planktonic stage in the mixed layer of the ocean and a benthic stage on the deep-sea ocean bottom. The habitat shift signal was recorded within a 150 µm width of otolith growth zone, which was too narrow to be clearly detected by mechanical drilling and conventional isotopic ratio mass spectrometry. However, variations down to −7‰ were found in δ¹⁸O_otolith profiles as the result of Cs²⁺ beam sputter in the core and larval portions of the otoliths. Carbon mapping by electron probe microanalyzer and staining by toluidine blue suggested abundant proteins existed in the areas with anomaly negative δ¹⁸O_otolith values, which cannot be interpreted as a habitat change but due to the isotopic fractionation by O emission from the proteins. These results implied that careful design and understanding of the chemical composition of the analytical areas or tracks on the heterogeneous otolith was essential for highly accurate and precise analysis.     

The oxygen and carbon isotope distribution in the Mediterranean water masses

The oxygen and carbon stable isotope compositions of the present-day Mediterranean waters have been measured in order to evaluate their variability, which is related to the specific climatic and hydrological conditions within the basin. The experimental equation between the δ¹⁸O value and the salinity of water, based on 300 measurements on surface, intermediate, and deep waters sampled during the VICOMED 2 and 3 cruises in the western, central and eastern Mediterranean, has a slope of 0.27, a value which is significantly lower than the slope of 0.45, as defined in the northeast Atlantic Ocean. This difference in the δ¹⁸O–salinity relationship, which occurs immediately in the Alboran basin, is basically a characteristic of the climatic regime of the Mediterranean, i.e., of an excess evaporation over fresh water input. The largest variations of these two parameters, δ¹⁸O of water and δ¹³C of ∑CO₂, are observed in the surface waters, mostly in the western Mediterranean. This evolution mirrors the progressive eastward restriction, which separates the less-evaporated and more-productive western basins from the more-evaporated and less-productive eastern basins. The intermediate waters constitute a homogeneous layer. However, their δ¹⁸O values decrease eastward by 0.35‰ at maximum, due to progressive dilution by mixing with overlying and underlying water masses; their δ¹³C values decrease also eastward by 0.35‰ at maximum, due to an increasing input of nutrients issued from the regeneration of sinking organic particles. The deep waters have similar δ¹⁸O values but slightly higher δ¹³C values (often by less than 0.1‰) than the overlying intermediate waters, indicating generally well ventilated conditions due to active winter convection.     

Diagenesis and origin of calcite cement in the Flemish Pass Basin sandstone reservoir (Upper Jurassic): Implications for porosity development

The Flemish Pass Basin is a deep-water basin located offshore on the continental passive margin of the Grand Banks, eastern Newfoundland, which is currently a hydrocarbon exploration target. The current study investigates the petrographic characteristics and origin of carbonate cements in the Ti-3 Member, a primary clastic reservoir interval of the Bodhrán Formation (Upper Jurassic) in the Flemish Pass Basin.The Ti-3 sandstones with average Q_86.0F_3.1R_10.9 contain various diagenetic minerals, including calcite, pyrite, quartz overgrowth, dolomite and siderite. Based on the volume of calcite cement, the investigated sandstones can be classified into (1) calcite-cemented intervals (>20% calcite), and (2) poorly calcite-cemented intervals (porous). Petrographic analysis shows that the dominant cement is intergranular poikilotopic (300–500 μm) calcite, which stared to form extensively at early diagenesis. The precipitation of calcite occured after feldspar leaching and was followed by corrosion of quartz grains. Intergranular calcite cement hosts all-liquid inclusions mainly in the crystal core, but rare primary two-phase (liquid and vapor) fluid inclusions in the rims ((with mean homogenization temperature (T_h) of 70.2 ± 4.9 °C and salinity estimates of 8.8 ± 1.2 eq. wt.% NaCl). The mean δ¹⁸O and δ¹³C isotopic compositions of the intergranular calcite are −8.3 ± 1.2‰, VPDB and −3.0 ± 1.3‰, VPDB, respectively; whereas, fracture-filling calcite has more depleted δ¹⁸O but similar δ¹³C values. The shale normalized rare earth element (REE_SN) patterns of calcite are generally parallel and exhibit slightly negative Ce anomalies and positive Eu anomalies. Fluid-inclusion gas ratios (CO₂/CH₄ and N₂/Ar) of calcite cement further confirms that diagenetic fluids originated from modified seawater. Combined evidence from petrographic, microthermometric and geochemical analyses suggest that (1) the intergranular calcite cement precipitated from diagenetic fluids of mixed marine and meteoric (riverine) waters in suboxic conditions; (2)the cement was sourced from the oxidation of organic matters and the dissolution of biogenic marine carbonates within sandstone beds or adjacent silty mudstones; and (3) the late phases of the intergranular and fracture-filling calcite cements were deposited from hot circulated basinal fluids.Calcite cementation acts as a main controlling factor on the reservoir quality in the Flemish Pass reservoir sandstones. Over 75% of initial porosity was lost due to the early calcite cementation. The development of secondary porosity (mostly enlarged, moldic pores) and throats by later calcite dissolution due to maturation of organic matters (e.g., hydrocarbon and coals), was the key process in improving the reservoir quality.     

台湾海峡铀的来源及保守性混合

Seawater samples are collected in the spring of 2013 from the Taiwan Strait for the analysis of uranium(U)concentrations and isotopic compositions using MC-ICP-MS, and the geochemical behavior patterns of U in the Taiwan Strait are then investigated. Average concentrations of individual U isotopes are(3.23±0.14) μg/kg for 238 U,(2.34±0.09)×10~(–2) μg/kg for ~(235)U and(2.05±0.07)×10~(–4) μg/kg for 234 U. Correspondingly, the U isotopic compositions are 155±18 for δ234U and 138±2 for 238U:235U. The U concentrations and isotopic ratios in the Taiwan Strait are similar to those of open ocean seawater, suggesting the dominance of the open ocean input to the strait's U pool.However, river input, as suggested by the slightly lower salinity than that of the open ocean, also affected the U concentrations and isotopic compositions in the strait. From a compilation of U concentrations in the Taiwan Strait and adjacent areas, including the Jiulong Estuary and Zhujiang Estuary, the Xiamen Bay and the northern South China Sea, a strong and significant relationship between U concentration and salinity [U:S; U=(0.093 4±0.002 4)S+(0.092 0±0.061 5)] is revealed, suggesting conservative mixing of U in the Taiwan Strait. To better understand the U geochemistry in the Taiwan Strait, a multiple endmembers mixing model is applied to estimate the contributions of potential sources. The open ocean seawater contributed 69%–95% of U in the Taiwan Strait, with river water approximately 2%, and dust deposition only around 0.13%. Therefore, the model results supported the open ocean input source and the conservative mixing behavior of U derived from the observation of U concentrations and isotopic ratios and U:S ratios. The sediment interstitial water may be an important source of U to the Taiwan Strait with a possible contribution of 3%–29%, consistent with previous investigations based on radium isotopes.However, further investigations are warranted to examine the U concentration in the sediment interstitial water and its input to the overlying seawater in the Taiwan Strait.     

A Porites lutea climate record from Sodwana Bay,South Africa

We report on a 41-year (winter 1970 to winter 2010) Porites lutea coral core climate record from Two-Mile Reef, Sodwana Bay, in the South-Western Indian Ocean. X-ray analysis, ultraviolet fluorescent photography and stable isotope (δ¹⁸O and δ¹³C) analysis revealed skeletal high-density, late-winter and low-density, late-summer bands with very little terrestrial humic input. An overall decrease in coral growth rate was seen over this period, possibly linked to global temperature and acidification trends. The stable isotopes δ¹⁸O and δ¹³C were predominantly out of phase, with calculated temperatures showing a slight increase over the 41-year period but with an overall decrease from 1994 to 2010. The insignificant ocean warming recorded in the coral supports the existence of a local, self-regulating, cold-water upwelling system from the adjacent shelf break and canyons that is potentially moderating coastal water temperature rise in Sodwana Bay.     

Characteristic analysis on oxygen isotopic tracer in the East China Sea and waters to east of the Ryūkyū-guntō

Samples of O isotopic tracer were mlleMed at Sections P₃,P₂₅,P_cM-t/2-E and P_CM-1/2.w in both the Fast China Sea and the area to the east of the Ryūkyū-gunto during October-November,1991.Analytical results of the δ¹⁸O are as follows: (1) In the Kuroshio area,the δ¹⁸O isolines are almost parallel to the 200 m isobath.The value of δ¹⁸O is negative and reaches minimum mt the main axis of the Kuroshio,and increases on both sides.(2) In the Taiwan Warm Current (TWC) area there is a high δ¹⁸O tongue extending to the northeast.(3) In the area near the coast,the distribution of δ¹⁸O isoline shows that the Changjiang River runoff diffuses seaward and the land-ocean isotopic effect from the nearshore to the offshore.(4) The values of δ¹⁸O are from -1.0×10^-3 to -0.5×10^-3 in the shelf.(5) There is a low mre of δ¹⁸O value(<-1.6×10^-3) at the 600 m layer in the Kuroshio area,which is quite in accord with the existence of a low salinity mre (S G 34.30) between the 600 and 800 m layers in the same area.Finally,the mrrelations of the δ¹⁸O with the salinity and temperature,the upwelling and so on are discussed.     

Radionuclides as tracers of water fronts in the South Indian Ocean—ANTARES IV Results

Anthropogenic ⁹⁰Sr, ^239,240Pu and ²⁴¹Am were used as tracers of water mass circulation in the Crozet Basin of the South Indian Ocean, represented by three main water fronts—Agulhas (AF), Subtropical (STF) and Subantarctic (SAF). Higher ⁹⁰Sr concentrations observed north of 43°S were due to the influence of AF and STF, which are associated with the south branch of the Subtropical gyre, which acts as a reservoir of radionuclides transported from the North to the South Indian Ocean. On the other hand, the region south of 43°S has been influenced by SAF, bringing to the Crozet Basin Antarctic waters with lower radionuclide concentrations. The ²³⁸Pu/^239,240Pu activity ratios observed in water and zooplankton samples indicated that, even 35 years after the injection of ²³⁸Pu to the Indian Ocean from the burn-up of the SNAP-9A satellite, the increased levels of ²³⁸Pu in surface water and zooplankton are still well visible. The radionuclide concentrations in seawater and their availability to zooplankton are responsible for the observed ²¹⁰Po, ^239,240Pu and ²⁴¹Am levels in zooplankton.     

The return of Pacific waters to the upper layers of the central Arctic Ocean

Temperature, salinity, and chemical measurements, including the nutrients silicic acid (Si), nitrate (NO₃), nitrite (NO₂), ammonium (NH₄), and phosphate (PO₄ or P), the oxygen isotopic composition of seawater (δ¹⁸O), and barium (Ba) concentrations were obtained from the central Arctic Ocean along transects radiating from the North Pole in early spring, 2000–2006. Stations that were reoccupied over this time period were grouped into five regions: from Ellesmere Island, (1) north along 70°W and (2) northwest along 90°W; near the North Pole, (3) on the Amundsen Basin flank and (4) directly over the Lomonosov Ridge; (5) through the Makarov Basin along 170–180°W. These regions had been shown by others to have undergone marked changes in water-mass assemblies in the early 1990s, but our time series tracer hydrographic data indicate a partial return of Pacific origin water within the mixed layer and the upper halocline layers beginning in 2003–2004. Back-trajectories derived from satellite-tracked ice buoys for these stations indicate that the upper levels of Pacific water in the central Arctic in 2004–2006 transited westward from the Bering Strait along the Siberian continental slope into the East Siberian Sea before entering the Transpolar Drift Stream (TPD). By 2004, the TPD shifted back from an alignment over the Alpha-Mendeleev Ridge toward the Lomonosov Ridge, as was characteristic prior to the early 1990s. At most stations occupied in 2006, a decrease in the Pacific influence was observed, both in the mixed layer and in the upper halocline, which suggests the Canadian branch of the TPD was shifting back toward North America. Clearly the system is more variable than has been previously appreciated.     

δ13C analyses of oceanic particulate organic matter

Seventy-nine δ¹³C analyses of oceanic particulate matter (> 0·μ) from semi-tropical (Gulf of Mexico, Caribbean and Atlantic) and polar (South Indian Ocean) waters showed that the carbon isotope composition of the particulate matter from the cold polar surface waters was lighter ($\text{?24·7}\text{to}\text{?26·0‰}$) than that from the surface in the semi-tropical regions ($\text{?19·8}\text{to}\text{?22·3 ‰}$), reflecting the temperature effect on the photosynthetic fixation of carbon. δ¹³C for deep samples (> 330 m) were generally more negative than the surface samples, except in some well-mixed polar areas.A difference both in organic carbon isotopic composition and percentage organic carbon in the POM and the tops of sediment cores was also apparent; a loss of approximately 95 % of incoming carbon and an increase in ¹³C of several per mille being observed during deposition of particulate matter. This indicates that after settling on the bottom there is extensive diagenesis of the POM by organisms, indicating the non-refractory nature of the organic matter.