Deep Indian Ocean Research Articles

Indian Ocean sedimentary calcium carbonate distribution and its implications for the glacial deep ocean circulation

The ocean plays a critical role in the global climate by modulating atmospheric CO2 via carbon exchange between the atmosphere and the ocean, a process tightly linked to ocean circulation changes. Ocean circulation transports atmospheric CO2 to the deep ocean via deep water formation and then transfers it with respired CO2, which regulate CaCO3 accumulation in marine sediments on seafloor. Compared to studies on the Atlantic and Pacific, much less has been done on past circulation changes in the Indian Ocean. This is partly due to the lack of systematic investigation on sub-basinal scales calcium carbonate (CaCO3) distributions and limited understanding of their controlling mechanisms in the Indian Ocean. Based on an extensive data compilation and a simple carbonate accumulation model, here we show that CaCO3 distributions in various basins of the Indian Ocean are mainly controlled by ocean circulation changes with productivity only playing a secondary role. Comparison of CaCO3 distributions between the Last Glacial Maximum (LGM) and the late Holocene suggests little change in deep water acidity between these times. If the source-water acidity remains unchanged through time, this finding suggests a vigorous ventilation of the deep Indian Ocean during the LGM. Given that the glacial oceans were likely more alkaline, the stability of deep Indian acidity must suggest greater storage of carbon (mainly as bicarbonate ion) during the LGM. Additionally, we find a very corrosive LGM deep water in the Southwest Indian Basin which may be attributed to intrusion of the Pacific Deep Water.

Distribution Pattern of the Benthic Meiofaunal Community Along the Depth Gradient of the Western Indian Continental Margin, Including the OMZ and Abyssal Plain

The continental margin harbors a variety of habitats that support incredible biodiversity and the function of their oceans' ecosystems. The meiofauna is considered a significant component of the benthic faunal community from the polar to the tropical regions. The meiofaunal community in the deep Indian Ocean, especially along the depth gradient, is poorly investigated. The present study aims to explore the benthic meiofaunal community structure along the depth gradients and its associated environment in the western Indian continental margin (WICM) and abyssal plain in the eastern Arabian Sea. Sediment samples were collected from seven different depths (111–3,918 m) along the WICM including the oxygen minimum zone (OMZ) and abyssal plain. A total of 22 taxa (groups) were encountered along the WICM. The nematodes (85%) were the most dominant taxa in all the depths, followed by copepods (11%), nauplii (5%), and polychaetes (1.36%). Our results suggest that (a) the organic matter has accumulated in OMZ sites; (b) a high amount of total organic carbon did not influence the meiofaunal density or biomass; (c) oxygen and depth gradients were significant drivers of the meiofaunal community, low levels of oxygen contributed to lower taxa diversity and density at 485 and 724 m depths; (d) a significant relationship of meiofaunal density and biomass with chloroplastic pigment equivalent (CPE) values indicates pelagic-benthic coupling. Copepods, nauplii, tanaidaceans, isopods, kinorhynchs, and cumaceans were affected by the low-oxygen conditions at the OMZ sites. Enhanced meiofaunal diversity, density, and biomass at deeper sites (non-OMZ-D) was attributed to increased abundance of copepods, nauplii, tanaidaceans, isopods, kinorhynchs, and cumaceans and were mostly concentrated on the surface sediment (0–4 cm) triggered by enhanced bottom-water oxygen and freshness of available food outside the OMZ except 3,918 m. Therefore, the present study showed the meiofaunal community pattern with respect to depth gradients and their related environmental changes, including the OMZ along the WICM and abyssal plain in the Arabian Sea. Based on this study, the future extent of these underestimated marine ecosystems must be considered a priority to understand its functional consequences.

The Sloshing and Diapycnal Meridional Overturning Circulations in the Indian Ocean

AbstractThe meridional overturning circulation (MOC) seasonality in the Indian Ocean is investigated with the ocean state estimate product ECCO v4r3. The vertical movements of water parcels are predominantly due to the heaving of the isopycnals all over the basin except off the western coast. Aided by the linear propagation equation of long baroclinic Rossby waves, the driving factor determining the strength of the seasonal MOC in the Indian Ocean is identified as the zonally integrated Ekman pumping anomaly, rather than the Ekman transport concluded in earlier studies. A new concept of sloshing MOC is proposed, and its difference with the classic Eulerian MOC leads to the so-called diapycnal MOC. The striking resemblance of the Eulerian and sloshing MOCs implies the seasonal variation of the Eulerian MOC in the Indian Ocean is a sloshing mode. The shallow overturning cells manifest themselves in the diapycnal MOC as the most remarkable structure. New perspectives on the upwelling branch of the shallow overturn in the Indian Ocean are offered based on diapycnal vertical velocity. The discrepancy among the observation-based estimates on the bottom inflow across 32°S of the basin is interpreted with the seasonal sloshing mode. Consequently, the “missing mixing” in the deep Indian Ocean is attributed to the overestimated diapycnal volume fluxes. Decomposition of meridional heat transport (MHT) into sloshing and diapycnal components clearly shows the dominant mechanism of MHT in the Indian Ocean in various seasons.

Radiocarbon Content of Dissolved Organic Carbon in the South Indian Ocean

AbstractWe report four profiles of the radiocarbon content of dissolved organic carbon (DOC) spanning the South Indian Ocean (SIO), ranging from the Polar Front (56°S) to the subtropics (29°S). Surface waters held mean DOC Δ14C values of −426 ± 6‰ (~4,400 14C years) at the Polar Front and DOC Δ14C values of −252 ± 22‰ (~2,000 14C years) in the subtropics. At depth, Circumpolar Deep Waters held DOC Δ14C values of −491 ± 13‰ (~5,400 years), while values in Indian Deep Water were more depleted, holding DOC Δ14C values of −503 ± 8‰ (~5,600 14C years). High‐salinity North Atlantic Deep Water intruding into the deep SIO had a distinctly less depleted DOC Δ14C value of −481 ± 8‰ (~5,100 14C years). We use multiple linear regression to assess the dynamics of DOC Δ14C values in the deep Indian Ocean, finding that their distribution is characteristic of water masses in that region.

Archaea Dominate the Ammonia-Oxidizing Community in Deep-Sea Sediments of the Eastern Indian Ocean-from the Equator to the Bay of Bengal.

Ammonia-oxidizing Archaea (AOA) and ammonia-oxidizing Bacteria (AOB) oxidize ammonia to nitrite, and therefore play essential roles in nitrification and global nitrogen cycling. To better understand the population structure and the distribution of AOA and AOB in the deep Eastern Indian Ocean (EIO), nine surface sediment samples (>3,300 m depth) were collected during the inter-monsoon Spring 2013. One sediment sample from the South China Sea (SCS; 2,510 m) was also included for comparison. The community composition, species richness, and diversity were characterized by clone libraries (total 1,238 clones), and higher diversity of archaeal amoA genes than bacterial amoA genes was observed in all analyzed samples. Real time qPCR analysis also demonstrated higher abundances (gene copy numbers) of archaeal amoA genes than bacterial amoA genes, and the ratios of AOA/AOB ranged from 1.42 to 8.49 among sites. In addition, unique and distinct clades were found in both reconstructed AOA and AOB phylogeny, suggesting the presence of niche-specific ammonia-oxidizing microorganisms in the EIO. The distribution pattern of both archaeal and bacterial amoA genes revealed by NMDS (non-metric multidimensional scaling) showed a distinct geographic separation of the sample from the SCS and most of the samples from the EIO following nitrogen gradients. Higher abundance and diversity of archaeal amoA genes indicated that AOA may play a more important role than AOB in the deep Indian Ocean. Environmental parameters shaping the distribution pattern of AOA were different from that of AOB, indicating distinct metabolic characteristics and/or adaptation mechanisms between AOA and AOB in the EIO, especially in deep-sea environments.

Interhemispheric controls on deep ocean circulation and carbon chemistry during the last two glacial cycles

AbstractChanges in ocean circulation structure, together with biological cycling, have been proposed for trapping carbon in the deep ocean during glacial periods of the Late Pleistocene, but uncertainty remains in the nature and timing of deep ocean circulation changes through glacial cycles. In this study, we use neodymium (Nd) and carbon isotopes from a deep Indian Ocean sediment core to reconstruct water mass mixing and carbon cycling in Circumpolar Deep Water over the past 250 thousand years, a period encompassing two full glacial cycles and including a range of orbital forcing. Building on recent studies, we use reductive sediment leaching supported by measurements on isolated phases (foraminifera and fish teeth) in order to obtain a robust seawater Nd isotope reconstruction. Neodymium isotopes record a changing North Atlantic Deep Water (NADW) component in the deep Indian Ocean that bears a striking resemblance to Northern Hemisphere climate records. In particular, we identify both an approximately in‐phase link to Northern Hemisphere summer insolation in the precession band and a longer‐term reduction of NADW contributions over the course of glacial cycles. The orbital timescale changes may record the influence of insolation forcing, for example via NADW temperature and/or Antarctic sea ice extent, on deep stratification and mixing in the Southern Ocean, leading to isolation of the global deep oceans from an NADW source during times of low Northern Hemisphere summer insolation. That evidence could support an active role for changing deep ocean circulation in carbon storage during glacial inceptions. However, mid‐depth water mass mixing and deep ocean carbon storage were largely decoupled within glacial periods, and a return to an interglacial‐like circulation state during marine isotope stage (MIS) 6.5 was accompanied by only minor changes in atmospheric CO2. Although a gradual reduction of NADW export through glacial periods may have produced slow climate feedbacks linked to the growth of Northern Hemisphere ice sheets, carbon cycling in the glacial ocean was instead more strongly linked to Southern Ocean processes.

Quaternary climate modulation of Pb isotopes in the deep Indian Ocean linked to the Himalayan chemical weathering

We use reductive sediment leaching to extract lead (Pb) from the authigenic fraction of marine sediments and reconstruct the Pb isotope evolution of the deep central Indian Ocean over the past 250 thousand years at ∼3 kyr resolution. Temporal variations define a binary mixing line that is consistent with data from ferromanganese nodules and which records mixing between two well-defined endmembers through time. The unradiogenic endmember appears to represent a widely-distributed Pb source, from mid-ocean ridges or possibly volcanic aerosols, while the radiogenic endmember coincides with the composition of Ganges–Brahmaputra river sediments that are indicative of the Himalayan weathering inputs. Glacial–interglacial Pb isotope variations are striking and can be explained by an enhancement of Himalayan contributions by two to three times during interglacial periods, indicating that climate modulates the supply of dissolved elements to the ocean. While these changes could accurately record variations in the continental chemical weathering flux in response to warmer and wetter conditions during interglacials, the relative proportions of Pb derived from the Ganges and Brahmaputra appear to have been constant through time. This observation may point towards particulate-dissolved interactions in the estuary or pro-delta as a buffer of short timescale variability in the composition (and potentially flux) of the fluvial inputs. In addition, the changes are recorded at 3800 m water depth, and with the lack of deep water formation in the Bay of Bengal, a mechanism to transfer such a signature into the deep ocean could either be reversible scavenging of dissolved Pb inputs and/or boundary exchange on the deep sea fan. Unless the mechanism transferring the Pb isotope signature into the deep ocean was itself highly sensitive to global climate cycles, and with the absence of a precessional signal in our Pb isotope data, we suggest that the Indian climate and its influence on basin-scale chemical weathering were strongly modulated by glacial versus interglacial boundary conditions.

Is the deep Indian Ocean MOC sustained by breaking internal waves?

The Indian Ocean hosts a vigorous basin‐scale overturning that constitutes one of the major deep upwelling branches of the global meridional overturning circulation (MOC). The extent to which the deep Indian Ocean MOC is sustained by breaking internal waves is assessed by quantifying and comparing the energetics of the overturning and those of the regional internal wave field. A range of published inverse estimates of the circulation across 32°S is used to assess the basin average buoyancy fluxes. The turbulent dissipation needed to sustain the MOC ranges between 0.17 ± 0.04 and 1.19 ± 0.17 TW, which is consistent with the estimated 0.35−0.26+1.04 TW dissipated by breaking internal waves, as inferred from observed fine structure. Both estimates of turbulent dissipation are consistent with the total energy input into the regional internal wave field (0.21−0.05+0.08TW) based on published estimates of energy conversion from winds, tides and geostrophic bottom flows. However, a discrepancy arises when comparing the energetics at different density levels. At mid‐ocean density levels (∼1000–3000 m) the dissipation of internal wave energy is found to be significantly smaller (factor 5–10) than the dissipation needed to sustain inverse estimates of the MOC. The uncertainty related to undersampling of internal wave breaking hot spots was analyzed and found to be small, which suggests that mixing processes other than wave breaking due to weak wave‐wave interactions, may be significant in the deep Indian Ocean.

The effect of topography-enhanced diapycnal mixing on ocean and atmospheric circulation and marine biogeochemistry

The impact of topographically catalysed diapycnal mixing on ocean and atmospheric circulation as well as marine biogeochemistry is studied using an earth system model of intermediate complexity. The results of a model run in which diapycnal mixing depends on seafloor roughness are compared to a control run that uses a simple depth-dependent parametrization for vertical background diffusivity. A third model run is conducted that uses the horizontal mean of the topographically catalysed mixing as vertical profile in order to distinguish between the overall effect of larger diffusivities and the spatial heterogeneity of the novel mixing parametrization. The new mixing scheme results in a strengthening of the deep overturning cell and enhances equatorial upwelling. Surface temperatures in the Southern Ocean increase by about 1 K (in the overall effect) whereas cooling of a similar magnitude in low latitudes is generated by the spatial heterogeneity of the mixing. The corresponding changes in the atmospheric circulation involve a weakening of the southern hemispheric Westerlies and a strengthening of the Walker circulation. Biogeochemical changes are dominated by an improved ventilation of the deep ocean from the south. Water mass ages decline significantly in the deep Indian Ocean and the deep North Pacific whereas oxygen increases in the two ocean basins. The representation of the global volume of water with an oxygen concentration lower than 90 μmol/kg in the model is improved using the topography catalysed mixing. Furthermore, primary production is stimulated in equatorial regions through increased upwelling of nutrients and reduced in the oligotrophic gyres.

Climate-driven body-size trends in the ostracod fauna of the deep Indian Ocean

Abstract: Body size is a common focus of macroevolutionary, macroecological and palaeontological investigations. Here, we document body-size evolution in 19 species-level ostracod lineages from the deep Indian Ocean (Deep Sea Drilling Program Site 253) over the past 40 myr. Body-size trajectories vary across taxa and time intervals, but most lineages (16/19) show net gains in body size. Because many modern ostracod taxa are larger in colder parts of their geographical range, we compared the timing and magnitude of these size changes to established Cenozoic deep-water cooling patterns confirmed through δ18O measurements of benthic foraminifera in the samples studied. These data show a significant negative correlation between size changes and temperature changes (ostracods get larger as temperatures get colder), and that systematic size increases only occur during intervals of sustained cooling. In addition, statistical support for an explicit temperature-tracking model exceeds that of purely directional evolution. We argue that this Cope’s Rule pattern is driven by secular changes in the environment, rather than any universal or intrinsic advantages to larger body sizes, and we note some difficulties in the attempts to link Cope’s Rule to observations made within a single generation.

Ocean temperature and salinity components of the Madden–Julian oscillation observed by Argo floats

New diagnostics of the Madden–Julian oscillation (MJO) cycle in ocean temperature and, for the first time, salinity are presented. The MJO composites are based on 4 years of gridded Argo float data from 2003 to 2006, and extend from the surface to 1,400 m depth in the tropical Indian and Pacific Oceans. The MJO surface salinity anomalies are consistent with precipitation minus evaporation fluxes in the Indian Ocean, and with anomalous zonal advection in the Pacific. The Argo sea surface temperature and thermocline depth anomalies are consistent with previous studies using other data sets. The near-surface density changes due to salinity are comparable to, and partially offset, those due to temperature, emphasising the importance of including salinity as well as temperature changes in mixed-layer modelling of tropical intraseasonal processes. The MJO-forced equatorial Kelvin wave that propagates along the thermocline in the Pacific extends down into the deep ocean, to at least 1,400 m. Coherent, statistically significant, MJO temperature and salinity anomalies are also present in the deep Indian Ocean.

The Zonal Dimension of the Indian Ocean Meridional Overturning Circulation

Abstract The three-dimensional structure of the meridional overturning circulation (MOC) in the deep Indian Ocean is investigated with an eddy-permitting ocean model. The amplitude of the modeled deep Indian Ocean MOC is 5.6 Sv (1 Sv ≡ 106 m3 s−1), a broadly realistic but somewhat weak overturning. Although the model parameterization of diapycnal mixing is inaccurate, the model’s short spinup allows the effective diapycnal velocity (the sum of model drift and the explicitly modeled diapycnal velocity) to resemble the true, real-ocean diapycnal velocity. For this reason, the model is able to recover the broad zonal asymmetry in the turbulent buoyancy flux that is suggested by observations. The model features a substantial deep, depth-reversing zonal circulation of nearly 50% of the MOC. The existence of this circulation, brought about by the zonally asymmetric distribution of diapycnal mixing, implies a much slower ventilation of the deep Indian Ocean (by a factor of 5–6) than would be in place without zonal interbasin exchanges. It is concluded that the zonal asymmetry in the distribution of diapycnal mixing must have a major impact on the deep Indian Ocean’s capacity to store and transform climatically significant physical and biogeochemical tracers.

Mantle 3He distribution and deep circulation in the Indian Ocean

The World Ocean Circulation Experiment Indian Ocean helium isotope data are mapped and features of intermediate and deep circulation are inferred and discussed. The 3He added to the deep Indian Ocean originates from (1) a strong source on the mid‐ocean ridge at about 19°S/65°E, (2) a source located in the Gulf of Aden in the northwestern Indian Ocean, (3) sources located in the convergent margins in the northeastern Indian Ocean, and (4) water imported from the Indonesian Seas. The main circulation features inferred from the 3He distribution include (1) deep (2000–3000 m) eastward flow in the central Indian Ocean, which overflows into the West Australian Basin through saddles in the Ninetyeast Ridge, (2) a deep (2000–3000 m) southwestward flow in the western Indian Ocean, and (3) influx of Banda Sea Intermediate Waters associated with the deep core (1000–1500 m) of the through flow from the Pacific Ocean. The large‐scale 3He distribution is consonant with the known pathways of deep and bottom water circulation in the Indian Ocean.

Detection of hydrothermal plumes along the Southeast Indian Ridge near the Amsterdam‐St. Paul Plateau

A new self‐contained profiling instrument, attached to 91 lowerings of waxcores and dredges along a 1600 km stretch of the Southeast Indian Ridge, found six sites having optical back‐scatter anomalies indicative of hydrothermal plumes from high‐temperature seafloor venting. At five other sites, back‐scatter anomalies were less distinct, and at the remaining sites, anomalies were absent. These are the first documented hydrothermal sites along this intermediate‐rate spreading center and are among the very few yet discovered in the Indian Ocean basin. Some profiles exhibit complex back‐scatter anomalies suggestive of multiple sources of seafloor discharge. At one site, near‐bottom temperature anomalies of ∼0.1°C and the recovery of a vent‐specific barnacle provide the first precise seafloor location of active venting in the deep Indian Ocean. Although this study is only reconnaissance in nature, venting along this portion of the SEIR appears to be significantly less than predicted by global correlations of plume incidence as a function of spreading rate. Nevertheless, these sites may be important as biological connections between distinct faunal assemblages of Atlantic and Pacific vent fields.

The distribution and mixing of Pacific water masses in the Indonesian Seas

The salinity and dissolved oxygen of water masses in the Indonesian Seas, from historical hydrographic data, are examined on isopycnal surfaces. We focus primarily on the Banda Sea, from which the bulk of the throughflow transport flows into the Indian Ocean. Dissolved oxygen proves to be a problematic conservative tracer in this region due to biological consumption in the upwelling regime of the Arafura Sea and the subsequent spreading of relatively low oxygen water over a broad area. The remaining analysis is thus restricted to salinity. We first consider a hypothesis of simple isopycnal advection and mixing between North and South Pacific low‐latitude western boundary current sources. Three regimes are apparent. The surface and upper thermocline layers, down to 25.8σθ, are too fresh to fit the hypothesis. Vertical mixing of surface precipitation and runoff excesses down into the water column must be invoked. Vertical mixing is also apparent on deeper isopycnals, below 27.0σθ, where a contribution from the deep Indian Ocean can be discerned. In between, in the lower thermocline, the θ‐S data are consistent with the hypothesis of purely isopycnal spreading, given a simple variation in the ratio of sources toward increasing South Pacific contribution with depth. In this regime the juxtaposition of sources leads to relatively strong gradients in water mass properties across the Banda Sea. This gradient translates into a difference in outflow characteristics between Timor and Ombai Straits, which appear to draw their waters, respectively, from the eastern and western Banda Sea. We then consider how the presence of vertical mixing modifies our inferred water mass ratio under a variety of boundary conditions. Vertical mixing effects are particularly important in the upper thermocline. In the Banda Sea, on all isopycnals down to σθ = 26.5, an increase in the degree of vertical mixing tends to decrease the relative importance of the North Pacific source. The sensitivity of the water mass ratio to degree of vertical mixing decreases with depth, and the tendency is reversed below 26.5σθ. In the Banda Sea, for plausible values of throughflow residence time and vertical diffusivity (λ = (κt)1/2 < 100 m), the North Pacific contributes 80–90% of the water mass at 26σθ, 50–60% at 26.5σθ, and only 10–30% at 27σθ. This large South Pacific contribution in the lower thermocline is supported by other studies.

A hydrographic section across the subtropical South Indian Ocean

Features of the water-property and circulation fields at the southern limit of the continentally bounded Indian Ocean are described on the basis of a transoceanic hydrographic section occupied along roughly Lat. 32°S by the R.R.S. Charles Darwin in November-December 1987. Primary observations consisted of 106 full-depth CTD/O 2 stations with discrete measurements of the concentrations of dissolved silica, phosphate and nitrate. The section lies in the southern part of the South Indian subtropical gyre; water-property features in the upper kilometer indicate that the northward interior flow is predominantly in the eastern half of the ocean there, consistent with the forcing pattern of wind-stress curl. The southward return flow is the Agulhas Current, whose transport at Lats 31–32°S is estimated as 85 × 10 6 m 3 s −1. Circumpolar Deep Water flows northward to fill the greater deep Indian Ocean by means of western-boundary currents in the Crozet Basin, Central Indian Basin and Perth Basin. North Atlantic Deep Water entering directly from the mid-latitude South Atlantic is almost entirely confined to the south-western Indian Ocean (Mozambique Basin, Natal Valley) by the topography of the Madagascar Ridge and Mozambique Channel. Geostrophic transport figures are presented based on a zero-velocity surface constructed along the section from the tracer-property evidence of where deep water was moving northward and where southward. Ekman transport, deduced from shipboard acoustic-Doppler profiler measurements, as well as synoptic and historical wind stress data, is found to be small (about 1 × 10 6 m 3 s −1 northward). Net transport (geostrophic and Ekman) across the section is estimated to be 7 × 10 6 m 3 s −1 southward, which implies a similarly sized Indonesia throughflow. Ambiquity in the geostrophic referencing scheme, and the magnitude of baroclinic eddy noise on the section, suggest this figure in uncertain by at least ±10 × 10 6 mm 3 s −1. The calculations obtain a figure for net transport of water below 2000 dbars of 27 × 10 6 m 3 s −1 northward, which specifies an average upwelling speed at the 2-km level north of 30°S of 6.9 × 10 −5 cm s −1. This estimate, perhaps uncertain by 20–30%, nonetheless contributes to growing evidence for an anomalously vigorous meridional circulation in the Indian Ocean. The associated calculations of heat and fresh water flux divergences demonstrate that the Indian Ocean thermohaline circulation essentially expresses a conversion of bottom and deep water to mid-depth thermocline, and near-surface water.

Nitrogen biogeochemical cycling in the northwestern Indian Ocean

The vertical distribution and fine scale structure of nitrate (NO3), nitrite (NO2), nitrous Oxide (N2O), phosphate (PO4), oxygen (O2) and chlorophyll α (chl α) were determined in the North Western Indian Ocean (NWIO) along a meridional section (67°E) from the Equator to the Gulf of Oman using an Autoanalyser for micromolar levels of nutrients, and chemiluminescence and gas chromatographic methods for nanomolar levels of NO3 and NO2 and N2O respectively. Three biogeochimically contrasting regimes were investigated: (1) the highly oligotrophic nutrient-depleted subtropical gyre; (2) the nonsoonal upwelling of nutrient-rich intermediate waters of the southeastern Arabian Coast; and (3) the denitrifying O2-depleted zone (ODZ; ca 150–1200 m depth) in the Arabian Sea. Concentrations of NO3 and NO2 were severely depleted in surface oligotrophic waters from the equator (average 43 and 3.6 nM respectively) to the subtropical gyre (12–15°N; average 13.3 and 2.0 nM respectively) with similar levels in the more stratified Gulf of Oman. Upwelling waters off Southern Arabia had three orders of magnitude higher NO3 levels, and throughout the NWIO, the calculated NO3-fuelled primary production appeared to be regulated by NO3 concentration. Existing Redfield ΔO2/ΔNO3 regeneration ratios (=9.1) previously derived for the deep Indian Ocean were confirmed (= 9.35) within the oxic upper layers of the NWIO. The “NO”-potential temperature relationship (Broecker, 1974 Earth and Planetary Science Letters, 23, 100–107) needed for the derivation of expected NO3 and NO3-deficits within the denitrifying ODZ were refined using an isopycnal, binary mixing model along the σθ = 26.6%, density layer to take into account the inflowing contribution of NO3-depleted Persian Gulf Water. Vertically integrated NO3-deficits increased northwards from 0.8 mol NO3-N m−2 at Sta. 2 (04°N), up to 6.49 mol NO3-N m−2 at Sta. 9, at the mouth of the Gulf of Oman, then decreased to 4.10 moles NO3-N m−2 toward Sta. 11, near the Straits of Hormuz. When averaged for the denitrification area of the Arabian Sea, this corresponds to a deficit of 118 Tg NO3-N. Adopting a recent Freon-11 based estimate of water residence time of 10 years (Olsonet al., 1993, Deep-Sea Research II, 40, 673–685) for the O2-depleted layer, we calculate an annual net denitrification flux of 11.9 Tg N to the atmosphere or approximately 10% of the global water column denitrification rates. Supersaturated N2O concentrations were found in both surface oxic and upwelling waters (up to 246%) and peaked at the base of the ODZ (up to 1264%) in the northern Arabian Sea. Both nitrification in oxic waters and denitrification in hypoxic layers can be invoked as sources of N2O. The inventory of excess N2O amounted to 2.55 ± 1.3 Tg N2O-N, corresponding to annual production of 0.26 ± 0.13 Tg from denitrification. This is comparable to earlier (Law and Owens, 1990, Nature, 346, 826–828) estimates of the ventilation flux of N2O (0.22–0.39 Tg yr−1) from the upwelling region of the Arabian Sea. The decadal response times for circulation, deoxygenation, denitrification and ventilation of the ODZ-derived N2O and CO2 greenhouse gases and their monsoonal coupling implies the Arabian Sea is a sensitive oceanic recorder of global climate change.

Hydrothermal 3He and manganese plumes at 19°29′S on the Central Indian Ridge

We report the discovery of a hydrothermal helium‐3 plume north of the Indian Triple Junction, on the Central Indian Ridge at 19°29′S. Water column δ3He anomalies show a spike of 33.8% at 2460 meter depth, i.e. the highest δ3He anomaly recorded so far in the Indian ocean.The helium‐3 data are correlated with manganese, giving a Mn/3He ratio of 700 mol/cm3 (3.8×104 g/cm3STP). The CH4/3He ratio deduced from available CH4/Mn data in the same area is around 7×106. These values are close to what is observed on water column plumes on the East Pacific Rise.On a regional scale, the 3He distribution over the Indian Triple Junction exhibits a strong δ3He increase at the Ridge crest, which appears to spread eastwards through the deep Indian ocean.

Ra-228 in the deep Indian Ocean

Samples collected from the deep Indian Ocean on the GEOSECS Indian Ocean Expedition and R.V. Vema cruise 34 reveal that horizontal processes control mixing in the lower 1 km of the water column. Intermediate 228Ra maxima occur throughout the deep water of the Indian Ocean. 228Ra becomes enriched in waters in contact with bottom sediments. This probably occurs as horizontally migrating water masses move from one basin to another. Thus 228Ra seems to trace the flow of water through the deep Indian Ocean. In the basins of the northwest Indian Ocean (Arabian, Madagascar and Crozet), an intermediate 228Ra maximum is associated with high Si waters originating in the north. A broad 228Ra maximum in the Wharton Basin is probably derived from a northward flowing western boundary current from the Antarctic. In the Central Indian Basin high Si waters spreading southward from the Bay of Bengal do not carry a significant 228Ra signal; but water from the south, which overrides this high Si water occurring at about 3700 m depth, does carry a significant 228Ra signal above and below 3700 m.

Transindian hydrographic section at Lat. 18°S: Property distributions and circulation in the South Indian Ocean

Water-property distributions and circulation in the South Indian Ocean are described and discussed with reference to color profiles of temperature, salinity, potential density (σθ,), and the concentrations of dissolved oxygen, silica, and phosphate as observed on a section of 59 closely spaced, deep hydrographic stations along Lat. 18°S, Madagascar to Australia, during July to August 1976. In the upper water, the origin and disposition of the salinity-maximum, oxygen-maximum, and salinity-minimum layers combine to demonstrate a general northward component of flow in the interior of the South Indian Ocean, penetrating to the depth of the intermediate oxygen minimum at about 1000 m. The northward volume transport is estimated geostrophically as about 20 × 106 m3 s−1, roughly consistent with an estimate of Sverdrup transport, based on the July mean wind-stress curl for the zone 16 to 20°S, of 16 × 106 m3 s−1. Southward flow along the east coast of Madagascar appears large enough to balance all the northward interior flow.The deep Indian Ocean is filled entirely from the Antarctic, but the Central Indian Ridge and Ninetyeast Ridge divide the flow into three separate circulation systems, each with its own western-boundary-current field, in the West Australian Basin, Central Indian Basin, and, at Lat. 18°S, Mascarene Basin. In the boundary-current systems of the Mascarene and West Australian basins, at least, the primary northward flow is accompanied by weak southward flow well off the bottom and well east of the boundary. Because the Central Indian Basin is closed off to the south at moderately deep levels, only the water at depths of 2000 to 3500 m in the boundary current there flows directly northward from the Antarctic; the lower deep water of that basin derives from the boundary current in the West Australian Basin by overflow across deep low-latitude saddles on the Ninetyeast Ridge. The net northward volume transport below 2000 m in each western-boundary-current system is estimated geostropically, on the basis of zero-velocity surfaces inferred from the tracer-property distributions, as: Mascarene Basin, 5 × 106 m3 s−1; West Australian Basin, 6 × 106 m3 s−1; and Central Indian Basin, 8 × 106 m3 s−1 (the latter figure including the very deep flow derived from the Ninetyeast Ridge overflow).