220 Deep-Sea Sediments - Patterns, Processes, and Stratigraphic Methods
gotten concept of mantle convection and constructed a model providing for a new sea
floor "every 300 to 400 million years", which, he suggested, "accounts for the relatively thin veneer of sediments on the ocean floor" (see Chap. 1).
Before we tum to a closer examination of the various types of deposits, we briefly
touch on the subject of how the sediment arrives at the sea floor, in the pelagic rain.
8.3 The Pelagic Rain
8.3.1 Importance of Fecal Transport. In the pelagic realm, the bulk of the sediment
arrives as a rain of particles. ("Marine snow" is a technical term for sinking aggregates, so we cannot use it in the general sense for sinking particles, although it would
be quite appropriate). The nature of this rain has been studied in recent years by
sediment traps, which in essence consist of funnels attached to moored buoys, anchored in various places in the open ocean. The funnels are positioned at various
depths. They are open at the top (with a hole 30 cm to 1 m in diameter), and they
have a collecting cup (or several cups) at the lower end. From such experiments it
was found that much of the pelagic rain - perhaps most of it - consists of fecal pellets
of various sizes and shapes, and in various states of disintegration. The pellets derive
from copepods, salps, krill (in the Antarctic), and other grazing organisms ( Fig. 6.2).
The accelerated sinking made possible by "fecal transport", and also by aggregate
formation, allows even the smallest of particles (wind-blown dust, coccoliths) to
arrive at the sea floor within a week or two. If left to settle on their own, they would
take years! In the case offine calcareous, siliceous, and organic particles, all of which
are subject to remineralization in the water column, they would not arrive at all on the
deep-sea floor.
8.3.2 Flux of Organic Matter. The proportion of the primary production that leaves
the photic zone (the export production; see Fig. 6.3) experiences substantial losses
during settling, despite fecal transport. Presumambly, this is due to break-up of pellets
and aggregates from scavenging and decay. The amount of organic mateial caught in
traps at various depths in the open ocean roughly decreases by the same factor as the
depth increases; that is, the amount caught at 1000 m is about one fifth of that trapped
at 200 m, given the same overlying productivity in the photic zone (Fig. 8.5).
The high export factor in the coastal ocean (Fig. 6.3) and the short distance to the
sea floor are two factors that enhance the burial of organic matter around continental
margins (Fig. 6.4). In comparison, rates in the deep sea are extremely low. Other
factors causing this contrast are seasonality and "fast stripping" (see below), as well
as high rates of burial of organic matter.
8.3.3 Seasonality of Flux. The larger particles (sand-size foraminifera and radiolarians) reach the deep-sea floor within I to 2 weeks, much as do the pellets. This rapid
settling of the pelagic rain means that seasonal fluctuations of productivity in surface
waters produce seasonal food supply, even in the abyssal environment. Thus, benthic
organisms on the deep-sea floor are subject to feast and famine, just like the plankton.
In high latitudes, seasonal variations in the particle rain is especially pronounced
(Fig. 8.6).
gotten concept of mantle convection and constructed a model providing for a new sea
floor "every 300 to 400 million years", which, he suggested, "accounts for the relatively thin veneer of sediments on the ocean floor" (see Chap. 1).
Before we tum to a closer examination of the various types of deposits, we briefly
touch on the subject of how the sediment arrives at the sea floor, in the pelagic rain.
8.3 The Pelagic Rain
8.3.1 Importance of Fecal Transport. In the pelagic realm, the bulk of the sediment
arrives as a rain of particles. ("Marine snow" is a technical term for sinking aggregates, so we cannot use it in the general sense for sinking particles, although it would
be quite appropriate). The nature of this rain has been studied in recent years by
sediment traps, which in essence consist of funnels attached to moored buoys, anchored in various places in the open ocean. The funnels are positioned at various
depths. They are open at the top (with a hole 30 cm to 1 m in diameter), and they
have a collecting cup (or several cups) at the lower end. From such experiments it
was found that much of the pelagic rain - perhaps most of it - consists of fecal pellets
of various sizes and shapes, and in various states of disintegration. The pellets derive
from copepods, salps, krill (in the Antarctic), and other grazing organisms ( Fig. 6.2).
The accelerated sinking made possible by "fecal transport", and also by aggregate
formation, allows even the smallest of particles (wind-blown dust, coccoliths) to
arrive at the sea floor within a week or two. If left to settle on their own, they would
take years! In the case offine calcareous, siliceous, and organic particles, all of which
are subject to remineralization in the water column, they would not arrive at all on the
deep-sea floor.
8.3.2 Flux of Organic Matter. The proportion of the primary production that leaves
the photic zone (the export production; see Fig. 6.3) experiences substantial losses
during settling, despite fecal transport. Presumambly, this is due to break-up of pellets
and aggregates from scavenging and decay. The amount of organic mateial caught in
traps at various depths in the open ocean roughly decreases by the same factor as the
depth increases; that is, the amount caught at 1000 m is about one fifth of that trapped
at 200 m, given the same overlying productivity in the photic zone (Fig. 8.5).
The high export factor in the coastal ocean (Fig. 6.3) and the short distance to the
sea floor are two factors that enhance the burial of organic matter around continental
margins (Fig. 6.4). In comparison, rates in the deep sea are extremely low. Other
factors causing this contrast are seasonality and "fast stripping" (see below), as well
as high rates of burial of organic matter.
8.3.3 Seasonality of Flux. The larger particles (sand-size foraminifera and radiolarians) reach the deep-sea floor within I to 2 weeks, much as do the pellets. This rapid
settling of the pelagic rain means that seasonal fluctuations of productivity in surface
waters produce seasonal food supply, even in the abyssal environment. Thus, benthic
organisms on the deep-sea floor are subject to feast and famine, just like the plankton.
In high latitudes, seasonal variations in the particle rain is especially pronounced
(Fig. 8.6).