Vertical and lateral flux on the continental slope off Pakistan: correlation of sediment core and trap results

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.

Decadal-resolution record of winter monsoon intensity over the last two millennia from planktic foraminiferal assemblages in the northeastern Arabian Sea

“…The upper continental slope off Pakistan (Figure 1) is characterized by a strong and permanent oxygen minimum zone (OMZ) between a water depth of 200 and 1200 m (Schulz et al, 1996; von Rad et al, 1995), resulting from mid-water oxygen consumption by high degradation rates of organic matter. Oxygen deficiency prevents post-depositional mixing of the sediment by burrowing organisms, which enables the formation and preservation of annually resolving varve-like laminated sediments (Lückge et al, 2001; Schulz and von Rad, 2014; Schulz et al, 1996, 2002). High sedimentation rates caused by high biological productivity as well as the lateral advection and resuspension of fine-grained terrigenous matter (Schulz and von Rad, 2014), combined with the lack of bioturbation, enables the identification of interannual and even seasonal signals (Kemp, 1996).…”

Sources of laminated sediments in the northeastern Arabian Sea off Pakistan and implications for sediment transport mechanisms during the late Holocene

“…Mineral particles that are carried as eolian dust and as river suspended matter into the surface layer form the lithogenic matter fraction of sinking particles in the ocean (Honjo et al, 1982). The sediment trap EPT-II collected particles at 590 m water depth, which is approximately 1400 m above the seafloor, so that we can assume that its major source is particulate matter export from the surface layer (Schulz and von Rad, 2014). The lithogenic matter fluxes show a bimodal pattern with the maximum in winter (January–February) and a secondary maximum in late summer (August–September; Figure 2b).…”

“…In this study, we investigated sediment samples obtained from sediment core SO130-275KL taken at a water depth of 782 m offshore Pakistan (Figure 1) close to the well-studied core SO90-56KA (Von Rad et al, 1999a). The aim of this study is to characterize the variability of lithogenic matter sources of the last 5 ka at the Makran coast by analysing grain-size distributions (GSDs) in varves and event layers and determine sediment sources by end-member modelling and comparison of results with local eolian dust, Hingol river bed sediments as well as sinking particles from a near-by sediment trap investigation (Schulz et al, 2002; Schulz and Von Rad, 2014). In combination with biogeochemical and mineralogical methods, we identify sources of organic matter, carbonate and lithogenic components and try to better understand the source and depositional processes of land-derived material.…”

Sea Level Change Controlled the Sedimentary Processes at the Makran Continental Margin Over the Past 13,000 yr

“…In the northern Arabian Sea of Pakistan, a strong oxygen minimum zone formed between a water depth of 200 and 1,200 m due to the rapid consumption of oxygen in the water column under zones of high primary productivity (von Rad et al, 1999). Additionally, varved sediments (mixed type, millimeter‐ to sub‐millimeter‐scale laminae) are preserved in the sediments of this area due to a lack of bioturbation under hypoxic conditions (Luckge et al, 2002; Schulz & von Rad, 2014). The laminations along the continental slope of Pakistan are characterized by alternations of light‐colored and organic carbon‐poor terrigenous laminae deposited mainly during the winter monsoon and dark and organic carbon‐rich layers (with abundant coccoliths and fish debris) deposited during the late summer monsoon when productivity is high (von Rad et al, 1999).…”

“…Meanwhile, rare but intense winter precipitation was dominant during the late Holocene. Laminated sediments have been observed, which are composed of terrestrial materials brought by winter rainfall and organic-rich materials deposited during the high-productivity late summer monsoon under strong oxygen minimum zone conditions without bioturbation (Luckge et al, 2002;Schulz & von Rad, 2014;von Rad et al, 1999).…”