ASSESSMENT OF TOTAL ORGANIC MATTER PRESERVATION IN MUDDY TIDAL FLATS

In order to assess the organic carbon preservation in muddy tidal flats, a variety of organic carbon measurements were a modern intertidal mudflat and two Pleistocene mudflat outcrops from Willapa Bay, Washington. The modern mudflat consists of a temperate-climate, high biomass, low sedimentation rate, fully marine oxygenated bottom waters, and oxygen depleted sediment. The Pleistocene outcrops represent two differently aged terraces, the youngest of which is 180 ka and the older is 350 ka. At the three sample sites, a detailed record of the preservation potential of total organic carbon (TOC) within temperate-climate muddy tidal flats can be evaluated. Measurements from loss on ignition analyses indicate that the modern mudflats contain abundant organic carbon, ranging from 2.0 to 6.0 wt%. In the comparable Pleistocene units the TOC is similar (3.0 to 6.0 wt %), suggesting minimal loss of organic carbon over time scales of >100 ka.

Mean grain size ranges for the modern ranged between 28-65 μm and 13-28 μm for the Pleistocene outcrops. In both the modern and Pleistocene datasets, no vertical pattern emerges where comparing TOC levels at the surface to those at depth. However, grain size and TOC appear to be linked, in that finer sediment contains more organic carbon. Based on TOC, and grain size, the semi-lithified rock and sediments would preserve in the geologic record as bituminous siltstone.

High TOC deposits are routinely associated with high rates of primary production and/or stagnant water bodies, which are associated with oxygen depletion of the bottom waters and sediment substrate. The mudflat deposits accompanying the Willapa Bay estuary system are found within a low sedimentation rate, high productivity, oxygenated bottom water setting and accumulate a high %TOC. This suggests that high rates of TOC production and sediment oxygen levels rather than bottom water oxygen levels are the key variables in the preservation of organic matter.