ALBERT

Description: Highlights • Prominent North Atlantic sea surface cooling and freshening of ~3.5 °C and ~0.7‰ during ~3.65–3.5 Ma. • Surface cooling and freshening inline with a decrease of benthic δ13C records from South Atlantic Ocean sites. • Surface cooling and freshening at the same time as sea ice extension in the Arctic Ocean. Abstract The North Atlantic Current (NAC) as part of the Atlantic Meridional Overturning Circulation (AMOC) is the major supplier of heat into the northern North Atlantic. Pliocene changes of AMOC strength were speculated to either have amplified or diminished the Northern Hemisphere Glaciation (NHG) 2.7 million years ago (Ma). However, from the North Atlantic, little evidence is known about AMOC changes at around 3.6 Ma. At this time the intensification of NHG started and culminated in the first major glacial M2 event at 3.3 Ma. To elaborate the climatic effects of variations in the NAC during this early stage of NHG, we here present millennial-scale resolved records from Deep Sea Drilling (DSDP) Site 610A in the northern North Atlantic. Our data of planktic foraminiferal Mg/Ca-based sea surface temperatures (SSTMg/Ca) and ice volume corrected salinity approximations (δ18OIVC-seawater) span the critical time period 4–3.3 Ma. From 3.65 to 3.5 Ma, we observe a distinct ~3.5 °C cooling and ~0.7‰ freshening of the sea surface, which we interpret to reflect a weakened NAC. At the same time Arctic sea ice grew and benthic δ13C in the South Atlantic suggest a weakened AMOC. We conclude that the weakened NAC in response to a sluggish AMOC fostered sea ice formation in the Arctic Ocean and high-latitude North Atlantic, which might have preconditioned the climate for subsequent continental glaciations.

Description: The Leeuwin Current flowing southward along West Australia is an important conduit for the poleward heat transport and interocean water exchange between the tropical and the subantarctic ocean areas. Its past development, and its relationship to Southern Ocean change and to Australian ecosystem response, however is largely unknown. We here reconstruct sea surface and thermocline temperatures and salinities from foraminiferal-based Mg/Ca and stable oxygen isotopes from offshore southwest and southeast Australia reflecting the Leeuwin Current dynamics over the last 60 kyrs. Its variability resembles the biomass burning development in Australasia from ~60–20 ka BP implying that climate-modulated changes related to the Leeuwin Current most likely affected Australian vegetational and fire regimes. In particular during ~60–43 ka BP, warmest thermocline temperatures point to a strongly developed Leeuwin Current during Antarctic cool periods when the Antarctic Circumpolar Current weakened. The pronounced centennial-scale variations in Leeuwin Current strength appear in line with the migrations of the Southern Hemisphere frontal system and are captured by prominent changes in the Australian megafauna biomass. We argue that the concerted action of a rapidly changing Leeuwin Current, the ecosystem response in Australia, and human interference since ~50 BP enhanced the ecological stress on the Australian megafauna until a tipping point was reached at ~43 ka BP, after which faunal recuperation no longer took place. While being weakest during the last glacial maximum, the deglacial Leeuwin Current intensified at times of poleward migrations of the Subtropical Front. During the Holocene, the thermocline off South Australia was considerably shallower compared to the short-term glacial and deglacial periods of Leeuwin Current intensification.

Description: Highlights • Tobago Basin subsurface temperature and salinity records from ~37 to 30 ka BP. • Cumulative tropical Atlantic upper ocean-atmosphere interactions crucial to functioning of North Brazil Current and Subtropical Gyre. • Rapid re-organizations of the tropical Atlantic upper ocean-atmosphere system at ~32.8 ka BP and ~ 21.8 ka BP. • Thresholds for southward dispersal of Salinity Maximum Water not set during the glacial time period of abrupt climate fluctuations. Abstract Ocean-atmosphere simulations corroborate the relationship between tropical Atlantic subsurface heat and salt storage driven by Salinity Maximum Water (SMW) and deglacial perturbations of the Atlantic Meridional Overturning Circulation (AMOC). Whether AMOC variability of the last glacial cycle affected SMW export into the tropical West Atlantic remained yet elusive. In order to assess the sensitivity of the tropical hydrography during abrupt and rapid glacial climatic and oceanic perturbations, we present century-resolving foraminifera-based subsurface (~200 m water depth) temperature and salinity reconstructions from Tobago Basin core M78/1–235-1. The proxy records were interpreted in terms of the closely related development of the North Brazil Current (NBC) and the North Atlantic Subtropical Gyre (STG) from ~37 to 30 ka BP, and in relation to their deglacial developments. Prior to ~32.8 ka BP, the cyclic variations in subsurface conditions were attributed to the NBC, which acted in line with a recurrent intensification and relaxation of the trade winds, subtle migrations of the Intertropical Convergence Zone, and the related moisture transport across Central America. Major and rapid re-organizations of the tropical Atlantic upper ocean-atmosphere system took place at ~32.8 ka BP and ~ 21.8 ka BP, unmirrored by major AMOC changes. Thresholds for sufficient heat and salinity accumulation in the STG to allow for formation and intensified subsurface dispersal of SMW were not achieved before late HS1, when AMOC weakening, according tropical heat backlog and surface warming by maximum Northern Hemisphere insolation acted together.

Description: Stable carbon (δ13C) and oxygen isotope (δ18O) were performed on the benthic foraminiferal species Uvigerina spp. The foraminiferal specimens were selected from piston core M78/1-235-1, which was recovered from Tobago Basin (11°36.53'N 60°57.86'W) from 852 m water depth during R/V Meteor Cruise 78/1. Sampling and analytical studies were carried out from 412.5 to 800 cm core depth at 2-4 cm spatial resolution. This dataset extends the existing benthic proxy records (Poggemann et al., 2017; doi:10.1029/2018PA003376) further back in time, now focusing on the glacial time period and the latest fully developed D/O events (~22.7-37 ka BP). The isotope analyses were performed in 2017 and 2019 on a Thermo Scientific MAT 253 mass spectrometer with an automated Kiel IV Carbonate Preparation Device. The proxy data provide stratigraphical information on the Tobago Basin core M78/1-235-1.

Description: Stable carbon (δ13C) and oxygen isotope (δ18O) and Mg/Ca-measurements were performed on the planktonic subsurface-dwelling foraminiferal species Globorotalia truncatulinoides. The foraminiferal specimens were selected from piston core M78/1-235-1, which was recovered from Tobago Basin (11°36.53'N 60°57.86'W) from 852 m water depth during R/V Meteor Cruise 78/1. Sampling and analytical studies were carried out from 630 to 800 cm core depth at 1-2 cm spatial resolution. This dataset extends the existing subsurface proxy records (Reißig et al., 2019; doi:10.1594/PANGAEA.897083) further back in time, now focusing on the time period of the latest fully developed D/O events (~30-37 ka BP). The isotope analyses were performed in 2019 on a Thermo Scientific MAT 253 mass spectrometer with an automated Kiel IV Carbonate Preparation Device. Trace metal analyses were performed on a VARIAN 720-ES Axial ICP-OES, a simultaneous, axial-viewing Inductively Coupled Plasma - Optical Emission Spectrometer coupled to a VARIAN SPS3 sample preparation system. The proxy data provide information on the Tobago Basin subsurface hydrography in relation to Western Boundary Current and North Atlantic Subtropical Gyre interdynamics.

Description: This dataset provides depth, age, and sediment reflectance data of piston core M78/1-235-1, which was recovered from Tobago Basin (11°36.53'N 60°57.86'W) from 852 m water depth. A MINOLTA CM-508d hand-held spectrophotometer was used during R/V Meteor Cruise 78/1 to measure the light reflectance (L*) of the sediment. The spectral reflectance is measured over a wavelength spectrum from 400 to 700 nm at a 10 nm pitch. Routine measurements were made at 1 cm intervals and automatically recorded using the SPECTRAMAGIC® v.2.11 software. The L*- dataset without age information is archived at doi:10.1594/PANGAEA.788740 (Bahr et al., 2012). The L* data provide stratigraphical information, and information on fluvial supply of terrigenous matter to the core location.

Description: Benthic foraminiferal d18O from IODP Site U1313 Uk37'-based SSTs from IODP Site U1313 Benthic foraminiferal d18O from DSDP Site 610 Uk37'-based SSTs from DSDP Site 610 TEX86-based SSTs from DSDP Site 610 Uk37'-based SSTs from DSDP Site 609

Description: Uk37'-based SSTs from IODP Site U1313