What happens to terrestrial organic matter in the ocean?

Abstract

Each year rivers transport approximately 0.25×10¹⁵ g of dissolved (<0.5 μm) organic carbon (DOC) and 0.15×10¹⁵ g of particulate (>0.5 μm) organic carbon (POC) from continents to the ocean. Global discharge of riverine DOC is sufficient alone to sustain turnover of the entire pool of organic carbon dissolved in seawater. Similarly, the input of terrestrial POC by rivers is adequate to supply all the organic carbon buried in marine sediments. Because riverine organic matter consists of highly degraded, nitrogen-poor remains of terrestrial organisms, it might be expected to suffer minimal respiration in the ocean. One of the biggest mysteries in the global carbon cycle, therefore, is that only a small fraction of the organic matter dissolved in seawater and preserved in marine sediments appears to be land-derived. Either our global budgets and distribution estimates are greatly in error, or both dissolved and particulate organic matter of terrestrial origin suffer rapid and remarkably extensive remineralization at sea. Although many uncertainties remain, recent studies provide growing evidence for destruction of both dissolved and particulate terrestrial organic matter in the ocean. The mechanisms by which these huge masses of organic materials might be so rapidly oxidized following discharge are largely unknown. This report focuses on the transport and transformations of land-derived organic matter in the ocean, highlighting recent research on the patterns and processes involved.

Introduction

The question of the fate of terrestrial organic matter (TOM) in the ocean has intrigued scientists for decades (Waksman, 1933; Bader, 1956; Handa, 1977; Deuser, 1988) and is key to understanding the global carbon cycle (Berner, 1989) and its anthropogenic perturbations (Siegenthaler and Sarmiento, 1993). TOM represents a huge source of reduced carbon (approximately 0.4×10¹⁵ g C yr⁻¹) to the marine environment (Schlesinger and Melack, 1981), integrates drainage basin processes, and incorporates bioactive elements whose cycling modulates the biosphere over geologic time (Meybeck, 1982; Thurman, 1985). Organic remains of terrestrial organisms preserved in marine sediments provide one of the most extensive records of life on the continents, whose continuously eroding surfaces are ephemeral over millennia and seldom preserved (Judson, 1968). Organic substances unique to land plants label continental debris from specific regions, imprinting marine deposits with information concerning proximity to continents, wind strength and other climatically-related variables (Summons, 1993). Successful environmental reconstructions and searches for fossil fuels often hinge on an understanding of processes controlling the distributions and destinies of TOM in fluvial/marine systems.

Many circumstances appear to favor efficient dispersion and sensitive detection of TOM in marine environments. Rivers alone discharge sufficient DOC to support the turnover of DOC throughout the ocean (Williams and Druffel, 1987) and enough POC to account for all the organic carbon now being buried in marine sediments (Berner, 1989; Hedges and Keil, 1995). Aeolian inputs of TOM are challenging to determine, but at open ocean sites appear to be comparable to the delivery rate of marine-derived organic matter to the seafloor (Zafiriou et al., 1985), and hence are geochemically significant (Peltzer and Gagosian, 1989). The major ultimate source of TOM is vascular plants, which are confined essentially to land and characteristically contain high concentrations of recalcitrant, nitrogen-free biomacromolecules such as lignin, tannin, suberin and cutin (de Leeuw and Largeau, 1993). These “higher” plants have distinctive stable carbon isotope compositions (Fry and Sherr, 1984) and produce a host of unique lipids (Simoneit, 1977; Peltzer and Gagosian, 1989) which can be used as sensitive biomarkers of terrestrial origin (Hedges, 1990; Prahl et al., 1994). Despite its large input, relative resistance to microbial degradation and multiple biomarkers, there is surprisingly little evidence that TOM persists as a major component of organic mixtures in sea-water and marine sediments (Hedges and Keil, 1995). Either our global budgets and tracer methods are terribly flawed, or the ocean has remarkable means of ridding itself of TOM while preserving organic matter of planktonic origin. This geochemical conundrum will be reviewed in the following discussion, along with possible methods for its resolution.