Storage and cycling of organic carbon and nutrients in Holocene coastal sediments
Geochemical analyses of Holocene coastal sediments from eastern England were made to
better understand the cycling of organic carbon and nutrients in the coastal zone in the past,
present and future. Sediments and peat were deposited in freshwater marshes, saltmarshes
and intertidal mud- and sand-flat environments that were much more extensive during the
Holocene than they are at present. The reduction in these areas, largely through human
activities, has decreased the potential annual accumulation and storage of organic carbon,
nitrogen and phosphorus associated with sediments. While the carbon and nitrogen contents
of modem intertidal environments are similar to Holocene intertidal areas, phosphorus is
enriched in modem sediments by up to a factor of two.
Budgets of nitrogen and phosphorus cycling in Fenland, eastern England, suggest that the
Holocene estuaries in this area were sinks of nutrients from the North Sea despite nitrogen
isotopic evidence suggesting that nitrogen buried in freshwater marshes was predominantly
terrestrially derived. The present-day estuaries are sources of nutrients to the North Sea as
riverine loads and atmospheric deposition are much higher than during the Holocene and
sedimentation is also greatly reduced.
The southern North Sea is probably autotrophic, in contrast to the coastal zone global average
which is heterotrophic. The major differences between these two areas are: 1) the global
coastal zone receives much greater loads of riverine particulate matter than the southern
North Sea, and 2) sedimentation in the global coastal zone occurs in large river deltas which
are absent from the relatively small European estuaries, thus much of the sediment supplied
to the North Sea is exported to the shelf edge. Approximately 4x 109 t C, 0.3 x 109 tN and
0.1 x 109 tP are currently stored in fine-grained Holocene sediments in the southern North
Sea coastal zone.