Christian Hübscher, Dirk Nürnberg

The beginning of the Mid-Pleistocene Transition (MPT) ~920 ka BP marked the expansion of northern hemisphere ice shields and caused a significant climate change in NW Europe. The MPT ended with the establishment of the 100 kyr ice age cyclicity at ~640 ka BP, due to orbital eccentricity changes. Previous studies explained the northern hemisphere cooling by cooling of sea-surface temperatures, increased sea-ice cover and/or changes in the Atlantic Meridional Overturning Circulation (AMOC) strength. We here discuss very-high resolution parametric echosounder (Parasound) imagery and sediment core analytics from a plastered drift at the eastern Campeche Bank (southern Gulf of Mexico), which was deposited under the influence of the Loop Current (LC). The LC transports warm tropical waters from the Caribbean into the Gulf via the Yucatan Channel. It is a key component of the Gulf Stream system, driving the ocean heat, salinity, and moisture transport towards the N Atlantic. The joint interpretation of reflection patterns, age constraints from color-scanning, foraminiferal stable oxygen isotopes, Sr isotope ratios (87Sr/86Sr) and core-seismic integration led to consistent conclusions about changes in LC strength across the MPT, thereby modulating the deep base level and the deposition of the plastered drift. The development of offlapping or onlapping plastered drifts, or the transition between the two termination patterns is best explained by changes in the depth of the relative deep base level and interpreted by changes in the flow regime.
Initially, the Middle Miocene to Pliocene closure of the Central American Seaway caused the onset and intensification of the LC and hence a deep base level fall. The sedimentary deposits from this phase have an offlapping prograding clinoform configuration, resembling a forced regression systems tract as is known from shelf areas. The deep base level fall caused sediment truncation above 500 m present day water depth. Below 500-550 m, the offlapping succession is overlain by sigmoidal and onlapping, transgressive systems tract like clinoforms. The transition from deep base level fall prior to the MPT to deep base level rise documents the weakening of the LC during the early MPT. After the MPT, the LC continued to weaken. The related reduction of heat transport from the Western Atlantic Warm Water Pool into the North Atlantic contributes to the further cooling of the northern hemisphere. Generally, the development of offlapping or onlapping plastered drifts or the transition between the two termination patterns can be explained by changes in the depth of the relative deep base level and interpreted by changes in the flow regime.