Phanerozoic growth of the epicontinental sedimentary reservoir: implications for long-term sea level change

Abstract:

Earth’s sedimentary carapace contains the largest surface-accessible reservoir of biogeochemically sensitive elements and contains several times more water than all of the present-day ice caps and glaciers combined. It is, therefore, widely recognized that on timescales of ~1 Myr, one of the most important factors governing the evolution of many Earth systems is the exchange of materials into and out of the sedimentary shell. Although it is rarely assumed that these rates of exchange are invariant, it is generally presumed that the sedimentary reservoir as a whole behaves as a single large, slowly cycling system in which erosion and sediment storage are balanced; hence the expectation that there is no net change in sediment volume. Here, using the Macrostrat database, which consists of surface and subsurface data for 1,474 locations as well as more than 700K geologic map-based polygons, we show that the sedimentary reservoir is best conceived of as multiple reservoirs with different intrinsic cycling rates determined by tectonic and environmental contexts of deposition. We also show that the volume of sediment stored on presently subaerially exposed North America has increased markedly during the Phanerozoic. Initiation of growth in the size of this epicontinental sedimentary reservoir is well recorded by the Great Unconformity, which separates predominately Precambrian-aged, low porosity crystalline and metamorphic basement rocks from overlying, more porous Cambrian and younger sedimentary deposits. Geologic map-based data from Eurasia and Australia suggest similar overall patterns globally. Thus, after burial of the subaerially exposed Great Unconformity surface by Cambrian-Ordovician sediments, the groundwater storage capacity of the continents increased by more than 15 million cubic km (~1% of present ocean volume). Subsequent burial by younger sedimentary deposits further increased epicontinental groundwater storage capacity to the ~130 million cubic km it contains today. Nearly monotonic growth in groundwater storage capacity may have accentuated tectonically-forced changes in sea level, may account for some of the observed long-term decline in the percentage of continental crust covered by shallow seas, and could have contributed to long-term trends in some related geochemical records.