Thursday, December 12, 2019

Dr. Brandon Curry, Senior Quaternary Geologist from the Illinois State Geological Survey
Title: From Source to Sink, Late Devonian Algal Cysts Delivered to the Gulf of Mexico During the Last Glaciation
Host: Isbell
Abstract:
A long-term biostratigraphic study in the Gulf of Mexico (GOM) documented reworked, late Devonian palynomorphs (tasmanitids) occurring in late Pleistocene (Wisconsin) marine sediments in the north-central GOM. Tasmanitids, which include the genera Tasmanites and Leiosphaeridia, are cysts of pelagic chlorophyllous algae. They occur, in abundance, as amber sand-size discs in late Pleistocene sediments. Nine GOM continental slope cores, covering a distance of 500 km, show that the average 14C age of the last occurrence (LO) of common, reworked tasmanitids is 17.8 ± 0.2 ka cal BP. The first occurrence (FO) of common tasmanitids, from fewer cores, has a radiocarbon age of28.5 ± 0.6 ka cal BP.
Tasmanitids occur in the Marine Isotope Stage 2 (MIS 2) including the Last Glacial Maximum (LGM). The geographic distribution of the late Devonian microfossils shows that the ancestral Mississippi River discharge site was the Mississippi Canyon. The nearly constant relative abundance of tasmanties, as well as their low density of about 1.1 glee, implies they were slowly deposited from suspension by meltwater-discharge plumes along with silt and clay. They occur in the Pigmy Basin and distal facies of Mississippi River submarine fan cores, collected during Deep Sea Drilling project Leg 96, and in slope cores used for engineering studies, which
crossed the Holocene/Pleistocene boundary. The tasmanitid occurrences in the GOM predate the major meltwater discharge (MWP 1-A) from glacial Lake Agassiz and, therefore, are not associated with this major event. We link the glacial history of the Last Glacial Maximum (LGM) to terrestrial and marine GOM records using a common proxy of reworked late Devonian palynomorphs along with 14C dating.