ALBERT

Description: Heinrich events are intervals of rapid iceberg-sourced freshwater release to the high latitude North Atlantic Ocean that punctuate late Pleistocene glacials. Delivery of fresh water to the main North Atlantic sites of deep water formation during Heinrich events may result in major disruption to the Atlantic Meridional Overturning Circulation (AMOC), however, the simple concept of an AMOC shutdown in response to each freshwater input has recently been shown to be overly simplistic. Here we present a new multi-proxy dataset spanning the last 41,000 years that resolves four Heinrich events at a classic mid-depth North Atlantic drill site, employing four independent geochemical tracers of water mass properties: boron/calcium, carbon and oxygen isotopes in foraminiferal calcite and neodymium isotopes in multiple substrates. We also report rare earth element distributions to investigate the fidelity by which neodymium isotopes record changes in water mass distribution in the northeast North Atlantic. Our data reveal distinct geochemical signatures for each Heinrich event, suggesting that the sites of fresh water delivery and/or rates of input played at least as important a role as the stage of the glacial cycle in which the fresh water was released. At no time during the last 41 kyr was the mid-depth northeast North Atlantic dominantly ventilated by southern-sourced water. Instead, we document persistent ventilation by Glacial North Atlantic Intermediate Water (GNAIW), albeit with variable properties signifying changes in supply from multiple contributing northern sources.

Description: Highlights • Records of seawater εNdεNd are highly unradiogenic across the OMT at Ceara Rise. • Strong influence of Amazon particulate Nd on seawater εNdεNd at Ceara Rise. • Point-sourced riverine Nd influences seawater far from continental shelf. • Regional sedimentary Nd flux can be confused with water mass mixing signals. Abstract Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and release from marine sediments along continental margins. This process, together with the short oceanic residence time of Nd, gives rise to pronounced spatial gradients in oceanic 143Nd/144Nd ratios (εNdεNd). However, we do not yet have a good understanding of the extent to which the influence of riverine point-source Nd supply can be distinguished from changes in mixing between different water masses in the marine geological record. This gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large-scale ocean water mass mixing, but also with important changes in continental hydroclimate and weathering. Here we present εNdεNd data for fossilised fish teeth, planktonic foraminifera, and the Fe–Mn oxyhydroxide and detrital fractions of sediments recovered from Ocean Drilling Project (ODP) Site 926 on Ceara Rise, situated approximately 800 km from the mouth of the River Amazon. Our records span the Mi-1 glaciation event during the Oligocene–Miocene transition (OMT; ∼23 Ma). We compare our εNdεNd records with data for ambient deep Atlantic northern and southern component waters to assess the influence of particulate input from the Amazon River on Nd in ancient deep waters at this site. εNdεNd values for all of our fish teeth, foraminifera, and Fe–Mn oxyhydroxide samples are extremely unradiogenic (εNd≈−15εNd≈−15); much lower than the εNdεNd for deep waters of modern or Oligocene–Miocene age from the North Atlantic (εNd≈−10εNd≈−10) and South Atlantic (εNd≈−8εNd≈−8). This finding suggests that partial dissolution of detrital particulate material from the Amazon (εNd≈−18εNd≈−18) strongly influences the εNdεNd values of deep waters at Ceara Rise across the OMT. We conclude that terrestrially derived inputs of Nd can affect εNdεNd values of deep water many hundreds of kilometres from source. Our results both underscore the need for care in reconstructing changes in large-scale oceanic water-mass mixing using sites proximal to major rivers, and highlight the potential of these marine archives for tracing changes in continental hydroclimate and weathering.

Description: We present Plio-Pleistocene records of sediment color, %CaCO3, foraminifer fragmentation, benthic carbon isotopes (δ13C) and radiogenic isotopes (Sr, Nd, Pb) of the terrigenous component from IODP Site U1313, a reoccupation of benchmark subtropical North Atlantic Ocean DSDP Site 607. We show that (inter)glacial cycles in sediment color and %CaCO3 pre-date major northern hemisphere glaciation and are unambiguously and consistently correlated to benthic oxygen isotopes back to 3.3 million years ago (Ma) and intermittently so probably back to the Miocene/Pliocene boundary. We show these lithological cycles to be driven by enhanced glacial fluxes of terrigenous material (eolian dust), not carbonate dissolution (the classic interpretation). Our radiogenic isotope data indicate a North American source for this dust (∼3.3–2.4 Ma) in keeping with the interpreted source of terrestrial plant wax-derived biomarkers deposited at Site U1313. Yet our data indicate a mid latitude provenance regardless of (inter)glacial state, a finding that is inconsistent with the biomarker-inferred importance of glaciogenic mechanisms of dust production and transport. Moreover, we find that the relation between the biomarker and lithogenic components of dust accumulation is distinctly non-linear. Both records show a jump in glacial rates of accumulation from Marine Isotope Stage, MIS, G6 (2.72 Ma) onwards but the amplitude of this signal is about 3–8 times greater for biomarkers than for dust and particularly extreme during MIS 100 (2.52 Ma). We conclude that North America shifted abruptly to a distinctly more arid glacial regime from MIS G6, but major shifts in glacial North American vegetation biomes and regional wind fields (exacerbated by the growth of a large Laurentide Ice Sheet during MIS 100) likely explain amplification of this signal in the biomarker records. Our findings are consistent with wetter-than-modern reconstructions of North American continental climate under the warm high CO2 conditions of the Early Pliocene but contrast with most model predictions for the response of the hydrological cycle to anthropogenic warming over the coming 50 years (poleward expansion of the subtropical dry zones).

Description: Highlights • We present LGC record of the Pb isotope composition Labrador Sea seawater. • These data can be used to track Laurentide Ice Sheet extent over Hudson Bay. • LIS retreat during the PGM was relatively fast compared to the LGM • The LIS first advanced significantly over Hudson Bay during MIS 4. • Our record does not support significant LIS retreat during MIS 3. Understanding the history of continental ice-sheet growth on North America, and in particular that of the Laurentide Ice Sheet (LIS), is important for palaeoclimate and sea-level reconstructions. Information on ice-sheet extent pre-dating the Last Glacial Maximum (LGM) is heavily reliant, though, on the outputs of numerical models underpinned by scant geological data. Important aspects of LIS history that remain unresolved include the timing of its collapse during Termination 2, the first time that it expanded significantly during the Last Glacial Cycle, and whether its volume was significantly reduced during marine isotope stage (MIS) 3. To address these issues and more, we present authigenic iron-manganese (Fe–Mn) oxyhydroxide-derived high-resolution records of Pb isotope data and associated rare earth element profiles for samples spanning the past ∼130 kyr from northwest North Atlantic Labrador Sea, IODP Site U1302/3. We use these new data to track chemical weathering intensity and solute flux to the Labrador Sea associated with LIS extent on the adjacent highly radiogenic (high Pb isotope composition) North American Superior Province (SP) craton since the Penultimate Glacial Maximum (PGM). Our new records show that relatively high (radiogenic) values characterise warm marine isotope stages (MIS) 5, 3 and 1 and the lowest (most unradiogenic) values occurred during cold stages MIS 6, 4 and 2. The radiogenic Pb isotope excursion associated with Termination 2 is short-lived relative to the one documented for Termination 1, suggesting that LIS retreat during the PGM was relatively fast compared to the LGM and that its collapse during the last interglacial occurred ∼125 ka. Highly radiogenic inputs to the Labrador Sea during MIS 5d-a, ∼116–71 ka, most likely reflect a spin-up in Labrador Current vigour, incipient glaciation and renewed glacial erosion of high grounds of the eastern SP craton by localised wet-based ice-caps. A large decrease in Pb isotope values towards unradiogenic LGM-like compositions between ∼75–65 ka across the MIS 5/4 transition likely reflects a slow-down in Labrador Current vigour, an increase in subaerial deposition of aeolian dust and a significant advance of the LIS across Hudson Bay caused a strong reduction or even abandonment of Pb sourcing from the SP. The relatively radiogenic Pb isotope composition of bottom-waters bathing our study site during MIS 3, 57–29 ka, is unlikely to support a recently proposed major reduction in LIS extent for this time. Instead, we argue these values are better explained by southern Greenland Ice Sheet retreat, increased chemical weathering of the Ketelidian Mobile Belt and subsequent Pb runoff from Greenland.

Description: Highlights: • We present a T5 record of the Pb isotope composition Labrador Sea seawater. • These data can be used to track Laurentide Ice Sheet (LIS) extent over Hudson Bay. • LIS retreat during T5 occurred over longer timescales than that for T2 and T1. • LIS deglaciation played important role in protracted nature of T5 sea-level rise. • Ice retreat during T1 may not be applicable template for older terminations in GIA modelling. Termination (T) 5, ∼424 ka, involved the biggest deglaciation of land-ice mass during the Quaternary. Warming and ice-sheet retreat during T5 led to an exceptionally long period of interglacial warmth known as Marine Isotope Stage (MIS) 11, ∼424–395 ka. A detailed understanding of the history of continental ice-sheet decay during T5 is required to disentangle regional contributions of ice-sheet retreat to sea-level rise (that range between ∼1 and 13 m above present day) and to correct it for glacio-isostatic adjustments (GIA). Yet little is known about the timing and magnitude of retreat during this time of the volumetrically most important continental ice sheet in the Northern Hemisphere, the Laurentide Ice Sheet (LIS). Here we present new authigenic Fe-Mn oxyhydroxide-derived high-resolution records of Pb isotope data and associated rare earth element profiles for samples spanning T5 from Labrador Sea IODP Site U1302/3. These records feature astronomically-paced radiogenic Pb isotope excursions that track increases in chemical weathering of North American bedrock and freshwater routing to the Labrador Sea via Hudson Straits associated with LIS retreat. Our records show that LIS retreat during T5 began 429. 2 ± 7.9 ka (2σ) and likely occurred over a longer timescale (by ∼10 to 5 kyr) than that observed for T2 and T1. They also show that Hudson Bay Ice Saddle collapse (and therefore LIS break-up) occurred ∼419 ± 4.7 ka (2σ), around the same time as best estimates of southern Greenland deglaciation, but ∼12 kyr before LIS deglaciation and the sea-level high-stand associated with the latter half of MIS 11 likely occurred. Our findings therefore highlight that ice-mass loss on North America likely played an important role in the seemingly protracted nature of T5 sea-level rise. A comparison of the deglaciation histories of the LIS and the southern Greenland Ice Sheet during T5, T2 and T1 also demonstrates that the well-constrained history of regional ice-sheet retreat during T1 is not always applicable as a template for older late Pleistocene terminations in GIA modelling.