Sea Level in Deep Time: The Record from Continental Margins

In order to predict future changes in sea level and shoreline location, it is vital to constrain the range past variability. Throughout the “Icehouse” period of the last ~30 million years, the changing volume of continental ice has been the principal driver of high-amplitude global sea-level change. The last ~1 million years have been dominated by ~100,000 year glacial/interglacial cycles and the sea-level rise since the last glacial maximum provides particularly valuable constraints on potential future rates of rise. However, earlier periods, notably the early Pliocene (~5.3-3 Ma) when CO₂ levels were similar to today’s, but temperatures were ~3° C and sea level ~25 m higher, provide glimpses of the possible future state of the earth system. In addition, the geological record shows that climate and sea level do not always respond linearly to forcing and that abrupt events can disrupt gradual trends: predictive computer models must be capable of reproducing all such conditions we know to have existed in the past.

A fundamental approach to investigating past changes in sea level involves investigation of the thick sediment accumulations beneath continental shelves and slopes. Continental margin sedimentary packages (sequences) and their bounding unconformities contain long records of global sea-level change. Complicating this record are basin subsidence, changes in the rate of sediment supply and other local processes that can superimpose their signatures on the preserved stratigraphy. Scientific ocean drilling of globally coordinated transects of boreholes across continental margins provides the best way to distinguish these effects and extract the sea-level signal. Such drilling targets the geological environment directly affected by sea-level change as the shoreline migrates back and forth across the continental shelf. Coring the resulting sequence stratigraphic record provides information on sediment ages, depositional environment and paleowater depths during sea-level cycles from coastal plain to outer shelf settings and spanning millions of years. Since boreholes provide information at only a few locations, integration of seismic imaging is vital to place drilling results within a two- and three-dimensional context in order to evaluate the influence on sequence architecture of along-margin changes in sediment input and basin morphology, as well as to provide paleogeomorphological constraints on sedimentary processes and paleoenvironments.