Abstract
Abstract. Due to the lack of bioturbation, the varve-laminated muds from the oxygen minimum zone (OMZ) off Pakistan provide a unique opportunity to precisely determine the vertical and lateral sediment fluxes in the nearshore part of the northeastern Arabian Sea. West of Karachi (Hab area), the results of two sediment trap stations (EPT and WPT) were correlated with 16 short sediment cores on a depth transect crossing the OMZ. The top of a distinct, either reddish- or light-gray silt layer, 210Pb-dated as AD 1905 ± 10, was used as an isochronous stratigraphic marker bed to calculate sediment accumulation rates. In one core, the red and gray layer were separated by a few (5–10) thin laminae. According to our varve model, this contributes < 10 years to the dating uncertainty, assuming that the different layers are almost synchronous. We directly compared the accumulation rates with the flux rates from the sediment traps that collected the settling material within the water column above. All traps on the steep Makran continental slope show exceptionally high, pulsed winter fluxes of up to 5000 mg m−2 d−1. Based on core results, the flux at the seafloor amounts to 4000 mg m−2 d−1 and agrees remarkably well with the bulk winter flux of material, as well as with the flux of the individual bulk components of organic carbon, calcium carbonate and opal. However, due to the extreme mass of remobilized matter, the high winter flux events exceeded the capacity of the shallow traps. Based on our comparisons, we argue that high-flux events must occur regularly during winter within the upper OMZ off Pakistan to explain the high accumulations rates. These show distribution patterns that are a negative function of water depth and distance from the shelf. Some of the sediment fractions show marked shifts in accumulation rates near the lower boundary of the OMZ. For instance, the flux of benthic foraminifera is lowered but stable below ~1200–1300 m. However, flux and sedimentation in the upper eastern Makran area are dominated by the large amount of laterally advected fine-grained material and by the pulsed nature of the resuspension events at the upper margin during winter.
Highlights
About 1 million km3 of shelf and slope waters of the global ocean are found to be permanently hypoxic, with more than half of that volume in the Indian Ocean (Helly and Levin, 2004)
We focus on the quantification of sediments accumulated along a depth transect in the Hab area (Fig. 1) between ∼ 200 and ∼ 2000 m water depth, and directly compare these accumulation rates with the limited numbers of flux estimates derived from sediment traps deployed in the overlying water column
A quantitative reconstruction of sediment fluxes using a combination of high-quality short sediment cores with sediment trap results leads to a better understanding of the roles of low-oxygen conditions and of the horizontal vs. vertical particle transport across the easternmost part of the upper Makran and the northern section of the Indus margin
Summary
About 1 million km of shelf and slope waters of the global ocean are found to be permanently hypoxic, with more than half of that volume in the Indian Ocean (Helly and Levin, 2004). After the pioneering study of von Stackelberg (1972) on the sedimentation along the Indian–Pakistan margin of the northern Arabian Sea, numerous campaigns and initiatives revealed the key role of the organic-rich, laminated sediments in studying the linkages between the production, pathways and preservation of marine organic matter under low bottomwater oxygen conditions (Paropkari et al, 1992; Pedersen et al, 1992; Calvert et al, 1995; Cowie et al, 1999; Keil and Cowie, 1999; van der Weijden et al, 1999; von Rad et al, 1999; Schulte et al, 2000; Suthhof et al, 2000; Cowie, 2005; Wiggert et al, 2005; Cowie and Levin, 2009). The northern Arabian Sea off Pakistan is characterized by a stable, distinct oxygen minimum zone (OMZ) between water depths of 200 m and about 1200 m that impinges on the continental slope. H. Schulz and U. von Rad: Vertical and lateral flux on the continental slope off Pakistan
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