FROM SOURCE TO SINK, LATE DEVONIAN ALGAL CYSTS (TASMANITES) DELIVERED TO THE GULF OF MEXICO DURING THE LAST GLACIATION: PART I, THE SINK

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-sized 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 cal. kyr B.P. The first occurrence (FO) of common tasmanitids, from fewer cores, has a radiocarbon age of 28.5 ± 0.6 cal. kyr B.P.

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 relative abundance of tasmanitids, as well as their low density of about 1.1 g/cc, implies they were slowly deposited from suspension by meltwater-discharge plumes along with silt and clay.

They occur in the Pigmy Basin and middle to 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.