Using The Anthropocene Marine Transgressive Sequence As A Marker For The Base Of The Anthropocene Epoch

The term “Anthropocene Marine Transgression” is used to delineate the beginning of a fourth major change in the rate of sea level rise during the Holocene period, which spans the last 12,700 years. This global rise in sea level is a direct response to anthropogenic sources of atmospheric CO² resulting in global warming and ice melt. The authors present a marine transgressive stratigraphic sequence from the coastal wetlands of Biscayne Bay, Florida, USA.

A series of historic aerial photographs document rapid mangrove tree retreat over fresh water wet parries dominated by periphyton beginning in approximately 1938. Mangrove produces organic-rich, marine-influenced sediment, and mangrove retreat is dependant upon salt water encroachment or tidal ingress to transport propagules, the reproductive organ of the mangrove. Such marine transgressive sequences, which are marine-influenced organic-rich strata overlying freshwater marl soils in this case, are observed globally as coastal wetlands retreat landward with sea level rise.

The rate of this global transgression varies regionally with differences in eustatic and isostatic conditions. Since this global transgression will leave a geological record similar to the numerous other transgressions used to define geological time periods in the past, the authors suggest consideration of the Anthropocene Marine Transgressive stratigraphic sequence as the basal marker of the proposed Anthropocene. Transgression is tied to anthropogenic-triggered global warming and, therefore, does not represent the first anthropogenic activities creating change to the natural environment but records the point where collective anthropogenic activities altered the climate sufficiently to increase the rate of ice melt and the resultant rise in sea level.

At the end of the Pleistocene (ice ages) era, the sea level was very low, perhaps 150 m below present, because a great deal of water was in the form of continental ice sheets. At the end of the Pleistocene, rapid ice melt resulted in a rapid rise of sea level, marking the beginning of the Holocene. The Early Holocene was a time of high rates of sea level rise > 10 mm yr^-1, too fast for coastal wetland or barrier island development because of overstep and submergence. Around 6 kyr ago, the rate of sea level rise decreased to between 2 and 3 mm yr^-1 resulting in the development of retreating coastal wetlands and barrier islands, which is referred to as the Middle Holocene.  The Late Holocene began ~ 3 kyr before the present when the rate of sea level rise slower to < 1 cm yr^-1 and coastlines stabilized.

The early 20th century the rate of sea level rise increased to 2 mm yr^-1 and further increased globally to 3.4 mm yr^-1 (NOAA) and regionally to 9 mm y^-1 in 2000 (Wdowinski et al. 2014) because of increasing rate of ice melt brought about global warming (IPPC 2013). This increase in the rate of sea level rise is associated with the development of transgressive stratigraphic sequence in subtropical coastal wetlands as described for coastal Biscayne Bay as well as numerous other sites globally.  The present rate of global sea level rise is 3.4 mm yr^-1, comparable with the rate during the Middle Holocene when coastlines were in retreat.

Our regional rate resulted in salt water encroachment and associated mangrove retreat of 500 m in 80 yr into previous freshwater wetlands and local inundation ponding. The present regional rate approximates the rates during the Early Holocene which is characterized by overstep (sea level rising too fast for coastal features like barrier islands and wetlands to develop) and submergence leaving a patchy record or depositional hiatus marking the base of the Anthropocene Marine Transgression and perhaps the proposed Anthropocene.

These findings are described in the article entitled, SE Saline Everglades transgressive sedimentation in response to historic acceleration in sea-level rise: A viable marker for the base of the Anthropocene? recently published in the Journal of Coastal Research. This work was conducted by John F. Meeder and Randall W. Parkinson from Florida International University.