B. Haake

Seasonality and interannual variability of particle-fluxes to the deep Arabian Sea

Abstract Long-term sediment trap studies have been carried out since 1986 at three locations in the western, central and eastern Arabian Sea. Here we present total and bulk component fluxes measured for 3 years at the central station and for 4 years at the western and eastern stations. Particulate fluxes to the deep sea are controlled by the monsoons with generally higher fluxes during the SW and NE monsoons and lower fluxes during the intermonsoon periods. The increase of particle fluxes occurs simultaneously with a drop in surface water temperature, induced by wind-or convective-mixing and an associated entrainment of nutrients into the euphotic zone. More than 50% of the annual particle fluxes to the deep sea occurs during the SW monsoon at the western location due to the prolonged influence of the monsoonal upwelling as indicated by increased biogenic carbonate and opal fluxes. However, the opal fluxes peak a month later than the carbonate fluxes. The delayed onset of opal flux peak appears to be controlled by the observed premonsoon silica distribution in the Arabian Sea, where the subsurface waters are silica depleted down to the thermocline at 150 m. At the central location particle fluxes are of similar magnitude during the SW and NE monsoons. The interannual variability of particle fluxes at the eastern location is determined by the NE monsoon. At the western and central locations, on the other hand, maximum interannual variability of fluxes occurs during the SW monsoon and particle fluxes were higher during years of stronger SW monsoon. The results further suggest that, apart from monsoon strength, geographic shifts of the area of maximum wind-stress may produce significant variabilities in particle fluxes to the deep ocean at the western Arabian Sea site.

Lithogenic fluxes to the deep Arabian Sea measured by sediment traps

Abstract Particle fluxes measured continously for one year at three locations in the Arabian Sea using time-series sediment traps show that lithogenic sedimentation processes are strongly coupled to biological processes. The vertical flux of lithogenic matter is controlled by episodic production and fluxes of biogenic matter. Illite and quartz are the dominant clay minerals in the traps at all three locations. Smectites generally range between 2 and 8%, but show higher fluxes up to 25% in the central and eastern Arabian Sea during the southwest monsoon period. Most of the river discharge is retained on the continental shelf, and less than 5% of the annual input of lithogenic material to the Arabian Sea is deposited in the deeper part as hemipelagic sediments.

Fluxes of amino acids and hexosamines to the deep Arabian Sea

Abstract Results of organic carbon, total nitrogen, amino acid and hexosamine analyses of samples collected during time-series sediment trap investigations in the Arabian Sea are presented. Samples were taken over a period of 1 1 2 years at two depths at each of three locations in the western, central and eastern part of the basin. Seasonal changes in amino acid contents and their spectral distributions show that degradation of organic matter in the water column is reduced during the monsoons, which are the high-flux periods in the western and central Arabian Sea. At the eastern site more degraded material of possibly recycled marine or terrestrial origin reaches the traps during the late summer peak fluxes. The results of hexosamine analyses suggest that bacterial biomass is relatively enriched on particles sinking in the water column and, to a larger extent, at the sediment-water interface. Decomposition between intermediate and deep water results in a loss of 30–40% of total organic carbon and more than 40% of amino acids. Comparison of the measured accumulation rates of organic carbon in sediment traps with those of organic carbon preserved in sediments show that more than 85% is lost before final burial in the sediments. Organic matter preservation in the Arabian Sea is higher than the average for the open ocean; this maybe due to the abundance of refractory organic matter of recycled marine or terrestrial origin.

Organic carbon removal in the sea: the continental connection

Abstract Time series sediment traps have been deployed since May 1986 in the Arabian Sea and since October 1987 in the Bay of Bengal. The results of one and a half years from the western, central, and eastern Arabian Sea and of one year from the northern Bay of Bengal show that particle flux patterns are related to the strong monsoon winds and heavy rains. Particle flux maxima in the Arabian Sea are mainly related to wind-induced deeper mixing and nutrient enrichment of surface waters during the SW and NE monsoons. Extremely high particle fluxes with high biogenic opal contents during the SW monsoon at the western location show that it is reached by nutrient-rich water from the near-shore upwelling centres. In the northern Bay of Bengal, particle flux maxima coincide with the period of maximum discharge of the Ganges and Brahmaputra rivers during which river plumes enriched in nutrients are advected into offshore areas. Additionally, the winds and river plumes supply enormous amounts of mineral matter during the periods of high productivity and high particle fluxes. The interaction between marine-biogenic material with eolian and fluviatile mineral particles plays a key role in sedimentation. The incorporation of mineral matter into biologically formed aggregates ensures their rapid sedimentation and thus enhances the removal of biologically fixed atmospheric carbon dioxide to the deep sea.

Downward flux of particulate fatty acids in the Central Arabian Sea

Abstract Particulate matter collected at 732 and 2914 m during a time-series sediment trap experiment (sampling interval 13 days) in the Central Arabian Sea (14°29′N, 64°46′E; water depth 4016 m) was analysed for its fatty acid and organic carbon contents. The sampling period covered the summer monsoon of 1986. Contents of organic carbon and fatty acid decrease with increasing particle fluxes. At 732 m fatty acids account for 0.6–4% of organic carbon. Particulate organic matter collected during high-productivity and high-flux periods exhibits signs of less intense degradation within the surface layers. Furthermore, loss rates of organic carbon and fatty acids between 732 and 2914 m increase with increasing particle flux, which suggests that water column degradation is intensified during this period. Selective degradation of fatty acids diminishes their contribution to organic carbon in the deeper trap (0.3–0.7%). Despite this, the seasonality in fatty acid flux is maintained down to the deep ocean. Differences in fatty acid composition between the two depths are indicative of the organisms involved in the degradation of particulate organic matter in the water column, especially bacteria. Biological alteration of sinking particulate matter appears to occur mainly on particles.

Sedimentation in the western Arabian Sea the role of coastal and open-ocean upwelling

Monsoon-induced coastal and open-ocean upwelling explain 84% of the variations of the organic carbon fluxes measured in the deep western Arabian Sea. In this paper, sea-level measurements, satellite-derived wind speeds, sea surface temperatures, and nutrient profiles are used to discern the relative importance of these factors on fluxes measured during nine years of continuous sediment trap deployments. This exercise shows: (i) the increase in fluxes observed during the initial stages of the SW monsoons are caused by open-ocean upwelling, which develops faster than the coastal upwelling; (ii) coastal upwelling triggers diatom blooms from nutrients from subsurface water and sediment resuspension and, more importantly, by injecting resting stages of diatoms back into the euphotic zone; (iii) silica depletion resulting from diatom blooms in laterally advecting water masses leads to a replacement of diatoms by other nitrate-limited organisms; (iv) organic carbon fluxes to the deep Arabian Sea increase in response to an intensification of both coastal and open-ocean upwelling; weak coastal upwelling and strong open-ocean upwelling also increase organic carbon fluxes. The varying dominance of their influence is reflected in the timing and the composition of the peak fluxes; (v) the link between organic carbon flux and monsoon strength is non-linear probably due to changes in the surface currents and to vigorous turbulence in the surface water during strong SW monsoons. These processes could reduce the organic carbon flux in the western Arabian Sea by about 65%.

Dynamics of dissolved organic carbon in the northwestern Indian Ocean

We report here the non-conservative behaviour of DOC in the northwestern Indian Ocean by studying this parameter together with other carbon and nitrogen components. This contrasts with earlier reports of conservative behaviour. Concentrations of DOC, 3–4 times higher than those reported earlier, were found to decrease northward from the equator. Total carbon dioxide (TCO2) increases in proportion of the oxygen utilized, thus revealing the dominant biological role in the carbon turnover. The CO2 added through dissolution of biogenic debris is found to decrease southward, in general. Decomposition of organic material contributes at least 64% to the CO2 addition that increases southward, the rest being from dissolution of skeletal material. Evidence is provided for the utilization of oxygen and nitrate for DOC oxidative decomposition. Accumulation of DOC without its complete oxidation to CO2 could be the main reason for the TCO2 decrease in southern Arabian Sea. Relationships of DOC with nitrification and denitrification processes show that the microbial population plays a major role in regulating the DOC contents in the seawater of this region. Consumption/decomposition by denitrifying bacteria and other micro-organisms responsible for nitrogen cycling in the sea are found to be intimately related to the DOC dynamics and are responsible for decreased DOC concentrations in the north. DOC accumulation in the southern Arabian Sea seems to facilitate bacterio-particulate aggregate formation and consequent nitrification, which results in excess nitrate. Application of a one-dimensional advection-diffusion model to the present data set provides evidence for the non-conservative nature of DOC in the Arabian Sea.

Amino acid, hexosamine and carbohydrate fluxes to the deep subarctic Pacific (Station P)

Abstract Sediment trap samples covering the period from September 1982 to September 1983 at Station P in the subarctic Pacific were analysed for organic carbon, nitrogen, amino acids, hexosamines and carbohydrates. One trap was deployed at 3800 m water depth for the whole period and a second trap was deployed at 1000 m from March to September 1983. Peaks of particle fluxes were observed in September–November 1982. May–June 1983 and July–September 1983. Organic compounds, used as indicators for organic matter sources and degradation, revealed that organic matter is generally least degraded during periods of maximum particle fluxes. During most of the year organic matter is derived from phytoplankton (mainly diatoms). In July to October concentrations of all measured organic compounds peak simultaneously at both trap depths, and amino acid and hexosamine fluxes are higher in the deeper trap. Spectral distributions of amino acids and hexosamines suggest that their increase is due to the addition of organic matter derived from copepods. In this case about 15% of the organic matter in the shallow trap and about 60% of the organic matter in the deep trap is contributed by copepods. The considerable enrichment of the more resistant hexosamines in the deep trap indicates that the copepods entering the traps are not active swimmers, but their decayed remains or molts.