108
J. I. Hedges
tively short time scale (millions of years). A weathering sink whose magnitude varies
with Oz concentration could help minimize atmospheric Oz fluctuations, but there is
a paucity of evidence for fossil carbon in modern marine sediments (Emerson et al.
1987), or for a build-up of sedimentary DC over the Phanerozoic (Broecker 1970). Thus,
it appears that oxidative weathering of sedimentary rocks is essentially complete in
to day's world, and hence has limited potential for counteracting upswings in Oz concentration (Walker 1974). All indications, therefore, are that the control valve for the
concentration of atmospheric Oz must operate on the source side during preservation
of sedimentary organic matter. Nevertheless, a responsive link to fluctuating sinks on
relatively remote continental surfaces appears necessary. In fact, research in the last
five years (Hedges and Keil1995) points toward an effective communication link between the continental sinks and sea floor sources of Oz, as well as an Oz-based negative feedback control mechanism on sedimentary organic matter preservation (Hedges
et al. 1999).
4.3
Organic Matter Preservation and Sediment Texture
Preservation in marine sediments is a rare fate for organic matter. Only one organic
carbon in -1000 ultimately escapes respiration and recycling back to COz. Although
the pelagic ocean accounts for over 80% of the surface area and primary production
of the global ocean (Fig. 4.2), over 90% of contemporary DC preservation occurs within
a relatively narrow band of shallow sediments flanking continental margins (Berner
1989; Hedges and Keil1995). The corresponding depth interval (-0-2000 m water
depth) includes deltas, continental shelves, and upper continental slopes, which together are generally characterized by the relatively rapid accumulation of organic-rich
sediments beneath oxygenated bottom waters. The observation that approximately
twice as much total organic matter is presently delivered from land to the ocean than
is preserved in all marine sediments (Smith and Mackenzie 1987) indicates that severe recycling mechanisms occur in the sea. Whatever processes are involved, they must
allow extensive remineralization of intrinsically resistant terrigenous organic matter
at sea and subsequent preservation of seemingly more labile marine-derived counterparts in marine sediments (Keil et al. 1997; Mayer et al. 1998). Thus, the key to understanding atmospheric Oz stabilization is to identify the mechanisms that modulate the minuscule leak of organic matter preserved in near-shore marine sediments
over geologic time.
Fig. 4.2. The percentage distribution of area, productivity and
organic carbon preservation
among anoxic, coastal and open
ocean marine waters. Although
the coastal ocean accounts for
only a small fraction of the
ocean's area and primary productivity, it is the predominant
site of sedimentary organic
carbon preservation
Area(%)
0.1%
9.9%
Production (%)
81.5%
0.5%
Preservation (%)
90' 8
6
%
3.1%
6.3%
1- . - o~e~o~a~ - - D ~o:sta~- -6 u-p:-el~g-]
J. I. Hedges
tively short time scale (millions of years). A weathering sink whose magnitude varies
with Oz concentration could help minimize atmospheric Oz fluctuations, but there is
a paucity of evidence for fossil carbon in modern marine sediments (Emerson et al.
1987), or for a build-up of sedimentary DC over the Phanerozoic (Broecker 1970). Thus,
it appears that oxidative weathering of sedimentary rocks is essentially complete in
to day's world, and hence has limited potential for counteracting upswings in Oz concentration (Walker 1974). All indications, therefore, are that the control valve for the
concentration of atmospheric Oz must operate on the source side during preservation
of sedimentary organic matter. Nevertheless, a responsive link to fluctuating sinks on
relatively remote continental surfaces appears necessary. In fact, research in the last
five years (Hedges and Keil1995) points toward an effective communication link between the continental sinks and sea floor sources of Oz, as well as an Oz-based negative feedback control mechanism on sedimentary organic matter preservation (Hedges
et al. 1999).
4.3
Organic Matter Preservation and Sediment Texture
Preservation in marine sediments is a rare fate for organic matter. Only one organic
carbon in -1000 ultimately escapes respiration and recycling back to COz. Although
the pelagic ocean accounts for over 80% of the surface area and primary production
of the global ocean (Fig. 4.2), over 90% of contemporary DC preservation occurs within
a relatively narrow band of shallow sediments flanking continental margins (Berner
1989; Hedges and Keil1995). The corresponding depth interval (-0-2000 m water
depth) includes deltas, continental shelves, and upper continental slopes, which together are generally characterized by the relatively rapid accumulation of organic-rich
sediments beneath oxygenated bottom waters. The observation that approximately
twice as much total organic matter is presently delivered from land to the ocean than
is preserved in all marine sediments (Smith and Mackenzie 1987) indicates that severe recycling mechanisms occur in the sea. Whatever processes are involved, they must
allow extensive remineralization of intrinsically resistant terrigenous organic matter
at sea and subsequent preservation of seemingly more labile marine-derived counterparts in marine sediments (Keil et al. 1997; Mayer et al. 1998). Thus, the key to understanding atmospheric Oz stabilization is to identify the mechanisms that modulate the minuscule leak of organic matter preserved in near-shore marine sediments
over geologic time.
Fig. 4.2. The percentage distribution of area, productivity and
organic carbon preservation
among anoxic, coastal and open
ocean marine waters. Although
the coastal ocean accounts for
only a small fraction of the
ocean's area and primary productivity, it is the predominant
site of sedimentary organic
carbon preservation
Area(%)
0.1%
9.9%
Production (%)
81.5%
0.5%
Preservation (%)
90' 8
6
%
3.1%
6.3%
1- . - o~e~o~a~ - - D ~o:sta~- -6 u-p:-el~g-]