Description: During the late Pliocene global climate changed drastically as the Northern Hemisphere glaciation (NHG) intensified. It remains poorly understood how the North Atlantic Current (NAC) changed in strength and position during this time interval. Such changes may alter the amount of northward heat transport and therefore have a large impact on climate in the circum-North Atlantic region and the growth of Northern Hemisphere ice sheets. Using the alkenone biomarker we reconstructed orbitally resolved sea surface temperature (SST) and productivity records at Integrated Ocean Drilling Project (IODP) Expedition 306 Site U1313 during the late Pliocene and early Pleistocene, 3.68–2.45 million years ago (Ma). Before 3.1 Ma, SSTs in the mid-latitude North Atlantic were up to 6 °C higher than the present and surface water productivity was low, indicating that an intense NAC transported warm, nutrient-poor surface waters northwards. Starting at 3.1 Ma, surface water characteristics changed drastically as the NHG intensified. During glacial periods at the end of the late Pliocene and beginning of the Pleistocene, SSTs decreased and surface water productivity in the mid-latitude North Atlantic increased, reflecting a weakened influence of the NAC at our site. At the same time the increase in surface productivity suggests that the Arctic Front (AF) reached down into the mid-latitudes. We propose that during the intensification of the NHG the NAC had an almost pure west to east flow direction in glacials and did not penetrate into the higher latitudes. The diminished northward heat transport would have led to a cooling of the higher latitudes, which may have encouraged the growth of large continental ice sheets in the Northern Hemisphere.

Description: The global ocean–climate system has been highly sensitive to the formation and advection of deep overflow water from the Nordic Seas as integral part of the Atlantic Meridional Overturning Circulation (AMOC) but its evolution over the Pliocene–Pleistocene global cooling is not fully understood. In particular, changes in the sources and mixing of prevailing deep waters that were involved in driving overturning throughout the Pliocene–Pleistocene climate transitions are not well constrained. Here we investigate the evolution of a substantial deep southward return overflow of the AMOC over the last 4 million years. We present new records of the bottom-water radiogenic neodymium isotope (ϵNd) variability obtained from three sediment cores (DSDP site 610 and ODP sites 980/981 and 900) at water depths between 2170 and 5050 m in the northeast Atlantic. We find that prior to the onset of major Northern Hemisphere glaciation (NHG) ∼3 million years ago (Ma), ϵNd values primarily oscillated between −9 and −11 at all sites, consistent with enhanced vertical mixing and weak stratification of the water masses during the warmer-than-today Pliocene period. From 2.7 Ma to ∼2.0 Ma, the ϵNd signatures of the water masses gradually became more distinct, which documents a significant advection of Nordic Seas overflow deep water coincident with the intensification of NHG. Most markedly, however, at ∼1.6 Ma the interglacial ϵNd signatures at sites 610 (2420 m water depth (w.d.)) and 980/981 (2170 m w.d.) synchronously and permanently shifted by 2 to 3 ϵNd units to less radiogenic values, respectively. Since then the difference between glacial and interglacial ϵNd values has been similar to the Late Quaternary at each site. A decrease of ∼2ϵNd units at 1.6 Ma was also recorded for the deepest water masses by site 900 (∼5050 m w.d.), which thereafter, however, evolved to more radiogenic values again until the present. This major ϵNd change across the 1.6 Ma transition reflects a significant reorganization of the overturning circulation in the northeast Atlantic paving the way for the more stratified water column with distinct water masses prevailing thereafter.

Description: Late Pliocene changes in the advection of Mediterranean Outflow Water (MOW) derivates were reconstructed at northeast Atlantic DSDP/ODP sites 548 and 982 and compared to records of WMDW at West Mediterranean Site 978. Neodymium isotope (εNd) values more positive than −10.5/−11 reflect diluted MOW derivates that spread almost continuously into the northeast Atlantic from 3.7 to 2.55 Ma, reaching Rockall Plateau Site 982 from 3.63 to 2.75 Ma. From 3.4 to 3.3 Ma average MOW temperature and salinity increased by 2°–4 °C and ~1 psu both at proximal Site 548 and distal Site 982. The rise implies a rise in flow strength, coeval with a long-term rise in both west Mediterranean Sea surface salinity by almost 2 psu and average bottom water salinity (BWS) by ~1 psu, despite inherent uncertainties in BWS estimates. The changes were linked with major Mediterranean aridification and a drop in African monsoon humidity. In contrast to model expectations, the rise in MOW salt discharge after 3.4 Ma did not translate into improved ventilation of North Atlantic Deep Water, since it possibly was too small to significantly influence Atlantic Meridional Overturning Circulation. Right after ~2.95 Ma, with the onset of major Northern Hemisphere Glaciation, long-term average bottom water temperature (BWT) and BWS at Site 548 dropped abruptly by ~5 °C and ~1–2 psu, in contrast to more distal Site 982, where BWT and BWS continued to oscillate at estimates of ~2 °C and 1.5–2.5 psu higher than today until ~2.6 Ma. We relate the small-scale changes both to a reduced MOW flow and to enhanced dilution by warmwaters of a strengthenedNorth Atlantic Current temporarily replacingMOWderivates at Rockall Plateau.

Description: ABSTRACT FINAL ID: PP13B-1829 This study focuses on changes in the North Atlantic Meridional Overturning Circulation (MOC) and aims to provide a detailed reconstruction of intermediate to deep water mass distribution and circulation in the North Atlantic over the most recent Pliocene global warm period from ~3.3 to 3.0 million years ago (Ma). During that 300-kyr interval, the global temperature was ~3°C warmer than today, which is predicted to be reached before the end of this century due to the anthropogenic climate change. This warm period is therefore considered to be one of the closest and the most recent analogue for future global climate. It is thus uniquely suited for a case study with the goal to better understand the oceanographic and climatic processes controlling and responding to the global warming. Changes in deep circulation in the northeast Atlantic during the late Pliocene warm period were studied at a suite of IODP/ODP drill sites by using combined εNd, stable isotope, and Mg/Ca-based bottom water temperature records. First results show that the Southern Ocean Water may have influenced substantial volumes of the deep North Atlantic reaching depths as shallower as ~2400 m during that time. On longer timescale, a first significant re-organization of the deep circulation in the North Atlantic towards the modern conditions appears to start after ~1.5 Ma. – This study is currently under progress based on further core transects in the northern North Atlantic in order to better understand how water masses were distributed, how ocean circulation changed, and in which state the North Atlantic MOC was during this key period of past climate.

Description: Changes in northern North Atlantic deep water circulation during the past 4 million years (Ma) were studied at a suite of five IODP/ODP sites at water depths from 2400 to about 5000 m. Benthic δ13C records at these sites oscillate in parallel around values lower than today by ~0.2 ‰ during interglacials and by ~0.7 ‰ during pronounced glacials of the late Pliocene warm period. However, Pliocene Mg/Ca-based deep-water temperatures were 2 to 3 °C higher than today during interglacials and near modern levels during glacial periods. The coeval changes in ventilation at a lower level than today may indicate a weaker difference between these warm water masses than today, which is probably caused by water mixing in the northern North Atlantic. This is corroborated by εNd bottom seawater values near -9 to -10 at all sites, which differs markedly from the modern situation characterized by clear differences in εNd signatures between the water masses at these sites. Accordingly, the lesser ventilated, warm, and mixed water masses are most likely the result of the weaker overturning in the northern North Atlantic during that time of global warmth [1]. After 1.6 Ma, benthic δ13C records show a gradually improving ventilation, with pronounced glacial/interglacial oscillations, which came close to the modern-to-late-Pleistocene levels. This ventilation change in deep water masses was coeval with a clear divergence in εNd of bottom water masses between the different sites towards modern signatures. This is apparently linked to a significant change in the sources of circulating deep waters in northern North Atlantic. Accordingly, the first reorganization of the deep circulation in northern North Atlantic towards the modern situation appears to have started only after ~1.6 Ma, most likely as a response to increases in the amplitude of the Earth’s obliquity cycle during that time [2]. [1] Haywood & Valdes (2004) Earth and Planetary Science Letters 218, 363-377. [2] Laskar et al. (1993) Astronomy and Astrophysics 270, 522-533.

Description: Ocean Drilling Program (ODP) Site 982 provided a key sediment section at Rockall Plateau for reconstructing northeast Atlantic paleoceanography and monitoring benthic δ18O stratigraphy over the late Pliocene to Quaternary onset of major Northern Hemisphere glaciation. A renewed hole-specific inspection of magnetostratigraphic reversals and the addition of epibenthic δ18O records for short Pliocene sections in holes 982A, B, and C, crossing core breaks in the δ18O record published for Hole 982B, now imply a major revision of composite core depths. After tuning to the orbitally tuned reference record LR04, the new composite δ18O record results in a hiatus, where the Kaena magnetic subchron might have been lost, and in a significant age reduction for all proxy records by 130 to 20 ky over the time span 3.2–2.7 million years ago (Ma). Our study demonstrates the general significance of reliable composite-depth scales and δ18O stratigraphies in ODP sediment records for generating ocean-wide correlations in paleoceanography. The new concept of age control makes the late Pliocene trends in SST (sea surface temperature) and atmospheric pCO2 at Site 982 more consistent with various paleoclimate trends published from elsewhere in the North Atlantic.