Upstream propagation of sea-level signals in fluvio-deltaic environments: Time lags, dampening, and the dynamics of the fluvial surface

Abstract:

Changes to sensitive fluvio-deltaic landscapes can significantly impact the stability of the ecosystems and economies they support. The sedimentary record of past environmental states can help refine predictions of the responses of fluvio-deltaic landscapes to changes in the rates of external forcings, e.g. sea-level rise. To reconstruct the processes that led to the present architecture of fluvial deltas, we need a theoretical framework for separating signals of allogenic (external) forcings from autogenic (internal) dynamics in the sedimentary record.

Previous numerical efforts suggest that changes in the relief and curvature of the fluvio-deltaic surface profile in response to sea-level cycles can result in geologically long-lived lags in the system’s response. In particular, the transition from a concave profile during a sea-level lowstand to a convex profile during a sea-level highstand requires a substantial sediment volume to be transported from the upper portion of the profile to the nearshore region. This can result in decreased sedimentation rates or erosion in the upstream portion of the fluvio-deltaic surface during sea-level rise. In contrast, the transition from convex to concave and higher relief profile during sea-level fall can cause increased sedimentation rates in upstream areas.

We used an experimental delta data-set from the Tulane Delta Basin to quantify spatial differences in the response of the deltaic surface that was exposed to high amplitude sea-level cycles under constant sediment supply and water discharge. We observed that the average rate of sedimentation in the upper portion of the profile is out phase by approximately half a period, with respect to the sea-level signal, whereas the sedimentation rate in the nearshore region is in phase. We also observed an increase in the relief and concavity of the strike-averaged surface during sea-level fall and a shift towards convexity and a mild relief during sea-level rise. Strike-averaged sedimentation patterns, separated into phases of sea-level rise and fall, also occur in tandem with changes in the geometric profile. We are currently working on coupling these experimental observations with our numerical modeling framework to better assess the implications of these results for the reconstruction of paleo–sea level change.