Deep thoughts about the shallow subsurface
Keywords: Geophysics.

Submerged marine terraces, formed dominantly during lowstands of sea-level, contain the convolved records of eustatic sea-level change and crustal vertical motion. The constraints on paleo sea-level and crustal vertical motion yielded from marine terrace studies have wide ranging implications for people living in the presence of a changing coastline. Detailed methods are developed for mapping precise paleo sea-level indicators from submerged marine terraces in the California Continental Borderland. 16 successive submerged marine terraces at Santa Catalina Island yield a subsidence rate of 0.2 mm/yr spanning 1.1Ma. E/V Nautilus's ROV Hercules recovered middle Pleistocene fossils from the deepest oldest terraces at Santa Catalina Island. Radiocarbon 14C ages of fossils recovered from the ~90 m terrace bracket the location of the transgressive surface, which corresponds to a change in clinoform steepness. Pilgrim and Kidney Banks have similar marine terraces that indicate 0.3 mm/yr subsidence for at least 0.35 Ma, and subsidence no faster than 0.12 mm/yr between 0.35 and 1.15 Ma. The incursion of humans into the Americas occurred along a narrow strip of coast bounded by rapidly retreating ice sheets and rising sea levels. With overland migration impossible due to rivers carrying large volumes of glacial melt, the rapid spread of humans down the coast of North America was probably accomplished in small watercraft by people who did not venture far inland. The prehistoric marine archaeological sites which are necessary to confirm the coastal route hypothesis are now under ~100 m of water, and remain undiscovered due to the difficulty and expense of marine operations, the vast search areas, and the likelihood of destruction of any potential site during sea-level rise. Methodologies for constraining coastal paleogeography via ROV exploration and multichannel seismic are applied to the identification of points of maximum discovery potential for paleo-coastal archaeological sites. The Last Glacial Maximum paleoshoreline is identified from high resolution dense 2D seismic data near Little Harbor at Santa Catalina Island, and via ROV exploration at Hercules Knoll. The globally distributed marine terrace record is our key to understanding the coupled response of the crust to redistribution of the hydrosphere. Coastlines of non-glaciated passive margins are mantled by uplifted marine terraces that suggest that they are uplifting, yet geodetic measurements suggest subsidence. The disagreement between marine terrace-derived and geodetically measured vertical motion rates along passive margins is an artifact of flexural isostacy, which deforms the coastline long after a change in sea-level. Flexural effects are not included in GIA approximation coastal vertical motion to maintain computational simplicity, yet they are a significant part of the coastal subsidence we subtract from GPS disciplined tide gauges. Application of the flexure correction to estimates of glacio-isostatic subsidence of the coastline decreases the modeled rate of coastal subsidence, thereby increasing the observed sea-level rise to equal the observable components to sea-level rise, bringing closure to the sea-level budget. One half of the world's marine terrace record is under water, and sub-sea imaging is key to describing and surveying these features. PEF interpolation , developed by Jon Claerbout, treats raster data as a field with zero mean and equal variance across the entire dataspace. By building statistics about data in an area with complete data, missing data values can be interpolated at higher accuracy than by other methods available today (e.g. Kriging). The PEF interpolator is capable of producing interpolated solutions that match the spatial-frequency content of the original data and better fool the eye than other interpolation algorithms, enabling better, more accurate imaging of the seafloor.