Lipid biomarker fluxes in the Arabian Sea, with a comparison to the equatorial Pacific Ocean

https://doi.org/10.1016/S0967-0645(02)00037-1Get rights and content

Abstract

Fluxes and distributions of organic carbon (OC) and lipid biomarkers were measured in the western Arabian Sea as a function of season, depth, and distance from the coast of Oman during the US JGOFS Arabian Sea Process Study in 1994–1995. A strong seasonal pattern in OC and lipid flux is related to the annual monsoon cycle in the western Arabian Sea, with the highest fluxes measured during the Southwest Monsoon. Fluxes were greatly attenuated with depth in the water column and in surface sediments as remineralization effectively consumed particulate organic matter. A comparison of water-column fluxes with OC and lipid accumulation rates in sediments confirms that the water–sediment interface is a “hot-spot” for organic matter degradation. Biomarker compositions also varied seasonally and with distance offshore, reflecting seasonal and spatial succession of their biological sources and their subsequent export through the water column. Degradation of OC and biomarkers was extremely efficient, with only a tiny fraction (<1%) of their water-column flux preserved in sediments, although a range of compound-specific degradation efficiencies was apparent. This intense degradation has strong implications for contemporary carbon cycling and for interpreting sediment records for paleoceanographic reconstructions.

Introduction

Intense monsoons in the Arabian Sea closely link highly predictable seasonal patterns of physical processes, biogeochemical cycles and climate (e.g., Luther et al., 1990; Prell et al., 1992; Smith et al., 1998a). Land–sea pressure gradients from seasonal heating of Asia in summer drive atmospheric circulation and alternately lead to persistent southwesterly winds of the Southwest Monsoon (SWM) in summer and northeasterly winds of the Northeast Monsoon (NEM) in winter, respectively. In summer, wind stress of the SWM forces upwelling of nutrient-rich deep-water and thinning of the mixed layer in the western Arabian Sea, stimulating primary production. Complex eddy circulation and topography produce conditions favorable for development and transport of cold filaments and mesoscale eddies that advect living biomass and detrital biogenic particles offshore (Brock and McClain, 1991; Flagg and Kim, 1998; Honjo et al., 1999; Kim et al., 2001). High productivity leads to enhanced export into deep waters, where remineralization of sinking organic matter sustains one of the world's largest and most intense open-ocean oxygen minimum zones. In winter, cooling and a weaker NEM results in a thickening of the mixed layer and a second, but weaker, pulse in productivity and export. Slack winds between SWM and NEM characterize the relatively unproductive spring intermonsoon (SI) and fall intermonsoon (FI) periods.

The dynamic meteorological and oceanographic conditions of the Arabian Sea have led to sediment-trap investigations by an Indo-German consortium since 1986 (Nair et al., 1989; Haake et al., 1993; Haake et al., 1996; Rixen et al., 2000). These studies have documented the spatial and temporal variability of biogenic and lithogenic particle flux and composition in intermediate and deep waters at three sites (west, central and east Arabian Sea), showing the close relationship between monsoonal forcing and particle flux over multi-year periods. Greater than 50% of the annual particle flux in the central Arabian Sea occurs during the SWM, and biogenic material dominates over lithogenic material. These studies also noted interannual variability in flux, lags between the timing of physical forcings and peak particle flux, and an offset between peak fluxes of carbonate and silica. About 85% of the variability in carbon flux can be attributed to monsoon-induced upwelling (there is disagreement regarding the importance of open-ocean upwelling, e.g., Rixen et al., 2000; Smith, 2001). Aspects of the organic geochemistry of sinking particulate matter in the Arabian Sea provide information on local sources and decomposition of organic substances, in particular amino acids and hexosamines (Haake et al., 1992; Rixen and Haake, 1993), carbohydrates (Haake et al., 1996) and fatty acids (Reemtsma et al., 1990). Results from the western Arabian Sea (WAST) site are most pertinent to the present investigation.

Using the Indo-German results as a basis for further investigation, our goal in the Arabian Sea Process Study (ASPS) was to characterize organic geochemical changes associated with the annual monsoon cycle in the western Arabian Sea in concert with other ASPS process studies. We sought to evaluate seasonal and spatial variations in the flux and composition of organic biochemicals and the relationship between these parameters and physical and biogeochemical processes that influence them. The questions we addressed included: How do fluxes and compositions of biochemicals vary over the monsoon cycles along a transect leading from the Oman coast to the central Arabian Sea? Does the composition of organic matter change as it sinks through the water column and accumulates in sediments, and what processes might be responsible? What fraction of biochemicals is preserved in sediments; how does this fraction vary with surface-water character; and what are the implications for paleoceanographic reconstructions? To address these questions, we determined elemental and biochemical fluxes and compositions in time-series sediment traps deployed over an annual cycle along the southern US JGOFS section (Fig. 1) from off the coast of Oman (MS-1) to the central Arabian Sea (MS-4). This paper describes lipid biomarker distributions in sinking particulate matter captured by these traps in the context of the spatio-temporal variability in physical and biogeochemical processes of the western Arabian Sea.

Several recent ASPS reports on sediment-trap studies provide further background specific to this study. Lee et al. (1998) compiled surface water-to-sediment organic carbon fluxes obtained from primary production measurements, water-column export determined by 234Th and sediment traps, and accumulation rates in underlying sediment for the ASPS southern section. These results provide a broad overview of organic carbon removal and remineralization in the western Arabian Sea. Honjo et al. (1999) described detailed records of seasonal and spatial variability in mass flux, and flux of organic and inorganic carbon, biogenic silica and lithogenic Al to the interior of the Arabian Sea obtained from moored sediment traps. Temporal and spatial variability in trap-derived fluxes are interpreted in terms of monsoon-driven biogeochemical events. Prahl et al. (2000) developed an annual record of biomarker export to the interior of the central Arabian Sea at 2200 m depth at site MS-4 (Fig. 1) in relation to biological conditions of the upper ocean. And finally, analysis of particulate material using solid state 13C-NMR (Hedges et al., 2001) and a new mechanistic model (Armstrong et al., 2002) both conclude that extensive (>98%) biodegradation is accompanied by minimal change in bulk organic matter composition, implying physical protection and ballasting of the organic fraction by mineral matrices.

Section snippets

Sediment traps and sediment coring

Sinking particulate matter was collected using moored sediment traps with indented-rotating sphere valves to exclude swimmers and with carousels for time-series collections (IRS-C traps; Peterson et al., 1993; http://boto.ocean.washington.edu/aog/traps/irs_traps.html). Three sets of traps were deployed on bottom-tethered US JGOFS moorings of Honjo and Dymond (see Fig. 2 of Honjo et al. (1999) for a schematic of the mooring array) at sites MS-1, MS-3, and MS-4 (Fig. 1). MS-1 (17°41′N, 58°51′E)

Diagnostic biomarkers

The analytical scheme used in this study identified and quantified ∼60 individual compounds in the neutral lipid fraction and ∼40 acids. The complete data set of fluxes is available on the US JGOFS Database (http://usjgofs.whoi.edu/PI-NOTES/arabian/Wakeham/sedtrap_lipid_raw.html). Predominant neutral lipid classes included C25–C31 n-alkanes, C25-highly branched isoprenoid (HBI) alkenes, C14–C28 n-alkanols, C26–C30 sterols, C28–C32 alkyl diols and alkyl keto-ols, and C37–C39 alkenones. Fatty

Meteorological forcing as a control on organic carbon cycling

Meteorological data obtained from the ASI buoy located near MS-4 (Weller et al., 1998) characterize the NEM, SI, SWM, and FI periods based on wind and temperature fields. Monsoon periods are characterized by reduced surface water temperatures that follow periods of elevated wind stress. The SWM begins in June as southwesterly winds strengthen about 3-fold over SI winds and surface water temperatures begin to decrease in response to offshore advection of colder, upwelled water. Continuous

Overview

Distributions of lipid biomarkers demonstrate that the strong physical forcings and ensuing biological cycles in the western Arabian Sea are manifested in complex spatial and temporal patterns of particulate organic matter flux and composition. Biomarker fluxes fluctuate in time and space as a reflection of variations in biological production and community structure and succession. Subsurface maxima in flux may result from lateral (rather than strictly vertical) transport of particulate

Acknowledgments

Our study could not have succeeded without the tremendous effort of the WHOI/Oregon State/Brown University sediment trap/coring consortium, including S. Honjo, S. Manganini, J. Dymond, C. Moser, K. Brooksforce, T. Gann, J. Billings, W. Prell and D. Murray, the Captain and crew of R/V T.G. Thompson, and the administrative skills of S. Kadar. Analytical assistance at Skidaway was provided by A. Bradley, J. Johnson, and A. Biersmith. Reviews by S. Smith, S. Rowland, T.K. Pease and two anonymous

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