Dinocyst assemblage constraints on oceanographic and atmospheric processes in the eastern equatorial Atlantic over the last 44 kyr

Abstract

A new 44 kyr long record of dinoflagellate (phytoplanktonic organisms) cysts (dinocysts) is presented from a marine sediment core collected on the Congolese margin with the aim of reconstructing past hydrological changes in the equatorial eastern Atlantic Ocean since Marine Isotopic Stage (MIS) 3. Our high-resolution dinocyst record indicates that significant temperature and moisture variations occurred across the glacial period, the last deglaciation and the Holocene. The use of specific dinocyst taxa, indicative of fluvial, upwelling and Benguela Current past environments for instance, provides insights into the main forcing mechanisms controlling palaeohydrological changes on orbital timescales. In particular, we are able, for the last 44 kyr, to correlate fluvial-sensitive taxa to monsoonal mechanisms related to precession minima-obliquity maxima combinations. While upwelling mechanisms appear as the main drivers for dinoflagellate productivity during MIS 2, dissolved nutrientenriched Congo River inputs to the ocean also played a significant role in promoting dinoflagellate productivity between approximately 15.5 and 5 ka BP. Finally, this highresolution dinocyst study permits us to precisely investigate the suborbital timing of the last glacial-interglacial termination, including an atypical warm and wet oceanic LGM signature, northern high-latitude abrupt climate change impacts in the equatorial eastern Atlantic, as well as a two-step decrease in moisture conditions during the Holocene at around 7-6 and 4-3.5 ka BP.

Palaeohydrological changes over the last 50 ky in the central Gulf of Cadiz: complex forcing mechanisms mixing multi-scale processes

“…glacial-interglacial conditions responsible for changing oceanic circulation, as a main driver for phytoplanktonic productivities. This relationship between obliquity maxima and enhanced marine productivities has furthermore recently been evidenced in the equatorial eastern Atlantic Ocean with dinocyst data (Hardy et al, 2016). MIS 3 corresponds to a general stronger velocity of denser MOW export (grain-size analysis; Fig.…”

“…Taxonomical diversity in study samples was quantified through a variety of statistical analyses using the "Past version 1.75b" software (Hammer et al, 2001); most of these indexes are explained in Harper (1999). Here, we calculated the number of taxa per sample (S); the dominance (D), which ranges from 0 (all taxa are equally present) to 1 (one taxon dominates the community completely); and Margalef's richness index: (S−1)/ln(n), where n is the number of individuals counted in each sample.…”

Dinoflagellate fossils: Geological and biological applications

“…Considering the last glacial cycle and its recurring millennial climate changes, dinocyst analyses carried out in subtropical to tropical North Atlantic latitudes shed light on significant paleohydrological changes from the western (Vink et al, 2001;Gonzáles et al, 2008) to the eastern Atlantic margins (southern Portugal: Turon et al, 2003;Eynaud et al, 2000;Penaud et al, 2011; western African margin: Marret and Turon, 1994;Marret et al, 2008;Holzwarth et al, 2010;Kim et al, 2010;Penaud et al, 2010Penaud et al, , 2011Penaud et al, , 2016Hardy et al, 2016Hardy et al, , 2018.…”

Diatom response to oceanographic and climatic changes in the Congo fan area, equatorial Atlantic Ocean, during the last 190ka BP

“…Three major sources of fan material can be distinguished: (1) dissolved and suspended supply from the Congo River consisting of products of weathering of the West African continental hinterland and river phytoplankton productivity, (2) atmospheric dust and (3) microorganism remains (Jansen, 1990). The sediments of the deep-sea fan system are therefore an important source of information for reconstructing the geological history of the margin (Savoye et al, 2009), the conditions on the continent (Gingele et al, 1998;Jahns et al, 1996;Dalibard et al, 2014), the relationship between atmospheric and oceanographic circulation (Schneider et al, 1994;Dupont et al, 1999), the interactions between the Congo River and the tropical Southern Atlantic circulation (Jansen and van Iperen, 1991;Uliana et al, 2002) as well as the variations of primary productivity on the Congo-Angola margin (Schneider et al, 1997;Dale et al, 2002;Marret et al, 2008Marret et al, , 2013Hardy et al, 2016).…”