Particle flux studies of sediment traps from the northern and equatorial Indian Ocean

The Asian monsoon system governs seasonality and fundamental environmental characteristics in the study area from which two distinct peculiarities are most notable: upwelling and convective mixing in the Arabian Sea and low surface salinity and stratification in the Bay of Bengal due to high riverine input and monsoonal precipitation. The respective oceanography sets the framework for nutrient availability and productivity. Upwelling ensures high nitrate concentration with temporal/spatial Si limitation; freshwater-induced stratification leads to reduced nitrogen input from the subsurface but Si enrichment in surface waters. Ultimately, both environments support high abundance of diatoms, which play a central role in the export of organic matter. It is speculated that, additional to eddy pumping, nitrogen fixation is a source of N in stratified waters and contributes to the low-d15N signal in sinking particles formed under riverine impact. Organic carbon fluxes are best correlated to opal but not to carbonate, which is explained by low foraminiferal carbonate fluxes within the river-impacted systems. This observation points to the necessity of differentiating between carbonate sources for carbon flux modeling. As evident from a compilation of previously published and new data on labile organic matter composition (amino acids and carbohydrates), organic matter fluxes are mainly driven by direct input from marine production, except the site off Pakistan where sedimentary input of (marine) organic matter is dominant during the NE monsoon. The explanation of apparently different organic carbon export efficiency calls for further investigations of, for example, food web structure and water column processes.

(Table 1) Uncompacted and compacted sedimentation rate values corrected for bedding attitudes at DSDP Leg 66 Holes

Sediment accumulation rates, computed using agesediment thickness curves obtained from DSDP cores, are rarely corrected for compaction or bedding attitude to better approximate true sediment accumulation rates (c.f. van Andel et al., 1975; Davies et al., 1977; and Whitman and Davies, 1979). Variations with depth in either of these factors can hinder interpreting relative rates of sedimentary processes associated with a particular depositional environment. This problem becomes particularly relevant for convergent margin sediments, which often display variable bedding attitudes and pronounced changes in porosity, bulk density, and other parameters related to the compaction process at shallow depth. These rapid shallow changes render correlation of sedimentation rates within a single transect of holes very difficult. Two techniques have been applied to data collected from a transect of holes along the southwestern Mexico continental margin, DSDP Leg 66 (Fig. 1), to correct sediment accumulation rates for variations in compaction and bedding attitude. These corrections should help resolve true fluctuations in accumulation rates and their implications regarding convergent margin processes.

Grain-size distribution, carbon and calcium carbonate at DSDP Leg 63 Holes

Sand-silt-clay distribution was determined at Scripps on samples collected at the time the cores were split and described. The sediment classification used here is that of Shepard (1954); sand, silt, and clay boundaries are determined on the basis of the Wentworth (1922) scale. Thus the sand, silt, and clay fractions are composed of particles whose diameters range from 2000 to 62.5 µm, 62.5 to 3.91 µm, and less than 3.91 µm, respectively. This classification is applied regardless of sediment type and origin.

Grain-size distribution and Folk's statistics at DSDP Leg 64 Holes

Grain size of 139 unconsolidated sediment samples from seven DSDP sites in the Guaymas Basin and the southeastern tip of the Baja California Peninsula was determined by sieve and pipette techniques. Shepard (1954) classification and Inman (1952) parameters correlation were used for all samples. Sediment texture ranged from sand to silty clay. On the basis of grain-size parameter, the sediments can be divided into three broad groups: (1) very fine sands and coarse silts; (2) medium- to very fine silts; and (3) clays and coarse silts.