A macro-micro model of sedimentary delta growth that accounts for biological processes
Abstract
Sediment mass balance as expressed by the Exner equation has proved to be a useful approach for modeling the average dynamics during the formation of a sedimentary delta. Whole-delta versions of such models consist of a balance between mineral sediment supply, sea-level rise, and subsidence. A simple model calculates the net sediment supplied in each time step and then positions the resulting sediment prism, with assumed constant geometric topset and foreset slopes, onto a subsiding basement. In many deltas, buried organic carbon in the form of peat makes up a significant fraction of the sediment column, but peat accumulation and degradation are typically not included in purely physical delta models. Here we extend current Exner models of delta growth, in particular the simple geometric prism model, and develop exploratory models that explicitly account for peat accumulation and degradation via plant growth, burial, and microbial processes influenced by salinity. We propose a set of ‘lumped’ models aimed at averaging the critical small scale biological processes as source and sink terms that are inserted into larger scale delta growth model. By analogy to previous models of solidification in the metals literature, we refer to this modeling style as Macro-Micro modeling. The basic lumped biological model first splits the delta into two regions: a near-shore region with high salt concentration and an inland region with a higher fresh water presence. The boundary between the regions is geometric and our model is constructed in such a way that the relative lengths of the near-shore and inland regions are controlled by subsidence and sea level changes. Our initial starting assumption is that the net rate at which soil organic carbon (SOC) is produced is constant throughout the delta. In the near-shore region, however, where the higher salt concentration stresses the system, we assume an additional SOC loss rate term proportional to the carbon content in the delta. Even in this highly simplified framework, the model suggests that carbon dynamics in the sediment column can significantly influence delta evolution. In a model with hinge subsidence but without a SOC component the shoreline monotonically reaches a steady position where the mineral flux balances the subsidence rate. When we include our proposed simple SOC source term the interactions between the subsidence and delta carbon content result in a shoreline that initially advances and retreats before monotonically approaching a steady position; we hypothesis that the initial advance-retreat cycle could represent an important time scale for the role of biological processes in delta building.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMEP43A0653L
- Keywords:
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- 0545 COMPUTATIONAL GEOPHYSICS / Modeling;
- 4805 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Biogeochemical cycles;
- processes;
- and modeling