MODELLING THE INFLUENCE OF SEA LEVEL RISE ON ORGANIC CARBON BURIAL

For a long time sedimenlotological, palaeontological and geochemical changes have been related to sea level changes. The relation between sea level and deposition of organic matter has been investigated particularly in the Mesozoic where economically import hydrocarbon source rocks were deposited under global anoxic events. Despite three decades of research on the correlation between sea level and various proxies, there have been little quantification of the first order influence that sea level change have on nutrient inventory and carbon burial.

Here we attempt to calculate burial of organic matter as a function of sea level rise with a biogeochemical model that explicitly consider area distribution as a function of depth in the ocean and phosphate liberated from coastal sediments during transgression. It is quantified how imposed sea level rise, comparable to observations from the Cretaceous, influence the phosphate cycle and organic carbon burial.

In the simplest form sea level rise lead to oxygenation of the ocean when the phosphate (P) flux to the ocean from land is held constant. Less new organic production (NP) in the euphotic zone is required to give the same total organic burial during higher sea level, because burial depend both on the length organic particles settle though the water column and the shallow shelf area. This dependence enhance the shallow burial under higher sea level for a given dissolved P concentration and new production. As a result the dissolved P and NP decrease to achieve steady state.

During transient sea level rise we find coastal erosion may increase the P flux to the ocean by 10 to 20%, in turn resulting in reduction of the oxygen concentration and a modest increase of the C:P burial ratio, which lead to small carbon isotope events (+0.5 to 1 permil). Only in some instances, if the ocean is sufficiently close to the "edge of anoxia", will the redox related feedbacks be strong enough to cause significant carbon isotope events. Such thresh-hold behaviour may explain the different conclusions reached when observations are correlated to eustatic sea level. We find that the correlation between sea level and large isotope events probably only indirectly is a consequence of the overall forcing the organic burial events (e.g. volcanism or long term oxygen cycle).