Gert Jan Reichart

Astronomical age constraints and extinction mechanisms of the Late Triassic Carnian crisis

The geological record contains evidence for numerous pronounced perturbations in the global carbon cycle, some of which are associated with mass extinction. In the Carnian (Late Triassic), evidence from sedimentology and fossil pollen points to a significant change in climate, resulting in biotic turnover, during a time termed the ‘Carnian Pluvial Episode’ (CPE). Evidence from the marine realm suggests a causal relationship between the CPE, a global ‘wet’ period, and the injection of light carbon into the atmosphere. Here we provide the first evidence from a terrestrial stratigraphic succession of at least five significant negative C-isotope excursions (CIE)’s through the CPE recorded in both bulk organic carbon and compound specific plant leaf waxes. Furthermore, construction of a floating astronomical timescale for 1.09 Ma of the Late Triassic, based on the recognition of 405 ka eccentricity cycles in elemental abundance and gamma ray (GR) data, allows for the estimation of a duration for the isotope excursion(s). Source mixing calculations reveal that the observed substantial shift(s) in δ13C was most likely caused by a combination of volcanic emissions, subsequent warming and the dissociation of methane clathrates.

A Saltier Glacial Mediterranean Outflow: A saltier Glacial Mediterranean Outflow

AbstractThe state of Atlantic Meridional Overturning Circulation (AMOC) is influenced by both thestrength and the location of the Mediterranean Outflow Water (MOW) plume in the Gulf of Cadiz. Toevaluate the influence of MOW on AMOC over deglaciations, precise and accurate salinity and temperaturereconstructions are needed. For this purpose, we measured Mg/Ca and clumped isotopes of several benthicforaminiferal species at Integrated Ocean Drilling Program Site U1390 in the Gulf of Cadiz. The clumpedisotope results of Cibicidoides pachyderma, Uvigerina mediterranea, and Pyrgo spp. are consistent betweenspecies and record no significant difference in Last Glacial Maximum to Holocene deep water temperature.Over the deglaciation, the Mg/Ca-based temperatures derived from U. mediterranea indicate three periods ofMOW absence at Site U1390. Mg/Ca-based temperatures of Hoeglundina elegans and C. pachyderma are onaverage 6°C too cold when compared to the present core-top temperature, which we explain by a carbonateion effect on these epibenthic species related to the high alkalinity of the MOW. Combining deep watertemperature estimates with the benthic oxygen isotope data and considering different relationshipsbetween seawater oxygen isotopes and salinity, we infer a salinity decrease of MOW by three to eight unitsover the deglaciation and four units during Sapropel 1, accounting for the global δ18O depletion due to thedecrease in ice volume. Our findings confirm that the Mediterranean Sea accumulates excess salt during aglacial low stand and suggest that this salt surged into the Atlantic over the deglaciation, presumably duringHeinrich Stadial 1.Plain Language Summary The Gulf Stream is slowing down because of the meltdown of theGreenland ice sheet. In the past, such a slowdown often resulted in a brief but quite extreme climatecooling in the Northern Hemisphere. Fortunately, the Gulf Stream would eventually speed up again forreasons that remain poorly understood. It is thought that the exchange of water between the Atlantic Oceanand the Mediterranean Sea through the Strait of Gibraltar plays an important role in bringing the Gulf Streamback to speed. In order to test this idea, we need to know the strength of the Atlantic-Mediterraneanexchange during times at which the Gulf Stream slowed down. Little shell-like organisms called benthicforaminifera, which live at the bottom of the ocean, record information about the properties of the water inwhich they grow within their shells. By analyzing a set of foraminifera living at a location close to the Strait ofGibraltar, we infer that it is indeed likely that the Atlantic-Mediterranean exchange changed significantlyduring a slowdown of the Gulf Stream. It is questionable whether or not the Gibraltar exchange will alsointensify due to the current melting of ice.back to speed. In order to test this idea, we need to know the strength of the Atlantic-Mediterraneanexchange during times at which the Gulf Stream slowed down. Little shell-like organisms called benthicforaminifera, which live at the bottom of the ocean, record information about the properties of the water inwhich they grow within their shells. By analyzing a set of foraminifera living at a location close to the Strait ofGibraltar, we infer that it is indeed likely that the Atlantic-Mediterranean exchange changed significantlyduring a slowdown of the Gulf Stream. It is questionable whether or not the Gibraltar exchange will alsointensify due to the current melting of ice.