Signals in (Ancient) Soils: How do Geomorphic Processes Affect the Mixing of Local and Catchment-Averaged Climate Signals in Sedimentary Basins?

Abstract:

The environmental proxy record from ancient fluvial successions can provide quantitative estimates of past climatic states; however, the spatial and temporal scales over which proxies integrate climatic information can span orders of magnitude and introduce significant uncertainties into paleoclimate reconstructions. Whereas recent work has made substantial inroads by quantifying how non-uniform sedimentation rates in Earth’s deep time record influences the quality of climatic reconstructions, potential biases introduced by proxy formation processes are incompletely understood. Deposited sediment contains geochemical signals from biomarkers, stable isotopes, soil geochemistry, and ecosystem variables; it holds information integrated over distances comparable to the size of the catchment feeding any point in the channel network and time intervals comparable to the age of the oldest material exposed to erosion in upstream areas. On floodplains, the inherited catchment-averaged geochemical signal in the deposited sediment is gradually overprinted by the local environmental signal during soil production.

Using physical experiments, we propose a theoretical framework for understanding how the kinematics of a distributive fluvial system interact with proxy formation timescales to influence the relative contributions of local or catchment-averaged paleoenvironmental signals to the proxy record of an alluvial basin. Specifically, we evaluate whether and to what degree the preservation of local versus basin-averaged signals is influenced by the ratio of water discharge to sediment discharge, base-level rise rate, and position within a source-to-sink mass-balance framework. Results show that the probability of creating and preserving a record of in situ paleoenvironmental conditions versus catchment-averaged conditions is: (1) increased with shorter proxy formation timescales, (2) increased with higher basin subsidence rates, (3) decreased with higher ratios of water discharge to sediment discharge, and (4) increased in the distal portions of distributive systems, where 75% or more of the sediment mass supplied to the basin has been extracted from transport via deposition.