The response of paralic peatlands to short-lived climatic or eustatic events

Coastal plains act as significant carbon sinks where they accumulate organic material in extensive peatlands. Coal-bearing stratigraphy thus provides a crucial paleoclimatic archive for carbon cycle dynamics over geological time. To better understand the rate of carbon uptake from the atmosphere into the terrestrial biosphere we turn to the geological record. Our aim is to investigate whether upstream climate forcing, or downstream climate-induced sea-level fluctuation acted as fundamental control on deposition in ancient paralic peatlands. Such potential allogenic controls need to be disentangled from autogenic forcing which is often prevalent in paralic successions. To filter out allogenic controls on coal-bearing stratigraphy we study the spatial extent, chronology (by means of magnetostratigraphy), isochronicity, and facies architecture of coal-bearing successions in the geological record.

Our work focusses on the Paleocene Fort Union Formation exposed in Eastern Montana and Western North Dakota (Williston Basin, USA). This formation is remarkable because facies associations remain strikingly uniform over 100’s of km’s distance, across proximal to distal transects. We focus on the lowermost Ludlow Member, exposed in the Little Missouri River Valley (ND) and across the northeastern flank of the Cedar Creek Anticline (MT). We use a combined approach of magnetostratigraphic correlation and sedimentological facies comparison to constrain the extent and temporal evolution of changing landscapes reflected in the studied stratigraphy. We show that distinct facies changes occurring in the Ludlow Member over stratigraphic thicknesses of only a few meters to a few tens of meters are laterally continuous and traceable over 100’s of km’s. Such findings have implications for understanding the sensitivity of peat-forming landscapes to short-lived climatic or eustatic changes and can ultimately inform us about the rates and volumes of carbon that gets sequestered in the terrestrial biosphere during distinct phases of changing climate or sea-level.