Land–sea coupling of early Pleistocene glacial cycles in the southern North Sea exhibit dominant Northern Hemisphere forcing

We assess the disputed phase relations between forcing and climatic response in the early Pleistocene with a spliced Gelasian (∼ 2.6–1.8 Ma) multi-proxy record from the southern North Sea basin. The cored sections couple climate evolution on both land and sea during the intensification of Northern Hemisphere glaciation (NHG) in NW Europe, providing the first well-constrained stratigraphic sequence of the classic terrestrial Praetiglian stage. Terrestrial signals were derived from the Eridanos paleoriver, a major fluvial system that contributed a large amount of freshwater to the northeast Atlantic. Due to its latitudinal position, the Eridanos catchment was likely affected by early Pleistocene NHG, leading to intermittent shutdown and reactivation of river flow and sediment transport. Here we apply organic geochemistry, palynology, carbonate isotope geochemistry, and seismostratigraphy to document both vegetation changes in the Eridanos catchment and regional surface water conditions and relate them to early Pleistocene glacial–interglacial cycles and relative sea level changes. Paleomagnetic and palynological data provide a solid integrated timeframe that ties the obliquity cycles, expressed in the borehole geophysical logs, to Marine Isotope Stages (MIS) 103 to 92, independently confirmed by a local benthic oxygen isotope record. Marine and terrestrial palynological and organic geochemical records provide high-resolution reconstructions of relative terrestrial and sea surface temperature (TT and SST), vegetation, relative sea level, and coastal influence.During the prominent cold stages MIS 98 and 96, as well as 94, the record indicates increased non-arboreal vegetation, low SST and TT, and low relative sea level. During the warm stages MIS 99, 97, and 95 we infer increased stratification of the water column together with a higher percentage of arboreal vegetation, high SST, and relative sea level maxima. The early Pleistocene distinct warm–cold alterations are synchronous between land and sea, but lead the relative sea level change by 3000–8000 years. The record provides evidence for a dominantly Northern Hemisphere-driven cooling that leads the glacial buildup and varies on the obliquity timescale. Southward migration of Arctic surface water masses during glacials, indicated by cool-water dinoflagellate cyst assemblages, is furthermore relevant for the discussion on the relation between the intensity of the Atlantic meridional overturning circulation and ice sheet growth

Causes and consequences of widespread ocean anoxia in the past

Oceanic oxygen concentrations vary widely in space and time. Since oxygen is essential for (most) life, oceanic oxygen concentrations influence the distribution and diversity of life in the marine realm. The surface ocean is rich in oxygen because it is in direct contact with the atmosphere. Deeper waters, however, lose oxygen with time, because it is consumed by (micro)organisms that feed on organic matter produced by plankton that sinks towards the sea floor. Oxygen-rich surface water has to be mixed downwards to supply the deep ocean with oxygen. If mixing is inhibited or the supply of organic matter to deeper waters intensifies (leading to increasing oxygen consumption), deeper water layers may become devoid of oxygen, hence development of anoxia. At present, oxygen concentrations are decreasing in many regions of the world’s oceans, due to human intervention. Much research aims to elucidate the mechanisms and time scales of this so-called deoxygenation, to better project future trends. Here, these issues are addressed by studying massive ocean anoxia during the greenhouse climate system in the mid-Cretaceous (Cenomanian-Turonian Oceanic Anoxic Event; OAE2; ~94 million years ago; with an approximate duration of 600.000 years) and a more recent anoxic period in the Eastern Mediterranean (Sapropel S1; 10.000-6.000 before present). More specifically, existing marine palynological (notably fossil remains of dinoflagellates, a group of unicellular plankton) and geochemical data were compiled and new data were generated for critical sediment cores to understand the key drivers responsible for ocean anoxia and the biosphere and climate feedbacks. The results of this study show that climate-driven changes in temperature and hydrology, have been of particular importance for the occurrence of widespread ocean anoxia during OAE2 and sapropel S1. This is illustrated by the strong influence of river discharge from the Nile on the formation of Eastern Mediterranean sapropel S1, which not only led to freshwater stratification-induced preservation of organic matter, but also contributed to enhanced production of organic matter by delivering nutrients for marine primary productivity, eventually leading to increased oxygen consumption. This mechanism was also shown to be responsible for organic matter deposition during OAE2 on flooded continental shelves. As a consequence of extreme warmth, the hydrological cycle accelerated. Enhanced precipitation and runoff ultimately led to freshwater stratification and enhanced marine primary productivity, culminating in widespread ocean anoxia and organic carbon sequestration. Interestingly, temporarily cooler and drier conditions during OAE2, correspond to a minimum in sedimentary organic carbon content. This minimum can be tracked throughout the, during OAE2, generally anoxic deep proto-North Atlantic basin, suggesting large-scale oxygenation of the water column. This again implies that temperature and hydrology were crucial factors in determining bottom water oxygen concentrations. This cooling event not only diminished organic carbon sequestration, but was also shown to drive equatorward migration of dinoflagellate species, showing that relatively small changes in temperature during the mid-Cretaceous super-greenhouse world had significant implications for biogeographical patterns

Biogeochemistry of the North Atlantic during oceanic anoxic event 2: role of changes in ocean circulation and phosphorus input

The geological record provides evidence for the periodic occurrence of water column anoxia and formation of organic-rich deposits in the North Atlantic Ocean during the mid-Cretaceous (hereafter called the proto-North Atlantic). Both changes in primary productivity and oceanic circulation likely played a role in the development of the low-oxygen conditions. Several studies suggest that an increased input of phosphorus from land initiated oceanic anoxic events (OAEs). Other proposed mechanisms invoke a vigorous up-welling system and an ocean circulation pattern that acted as a trap for nutrients from the Pacific Ocean.Here, we use a detailed biogeochemical box model for the proto-North Atlantic to analyse under what conditions anoxia could have developed during OAE2 (94 Ma). The model explicitly describes the coupled water, carbon, oxygen and phosphorus cycles for the deep basin and continental shelves. In our simulations, we assume the vigorous water circulation from a recent regional ocean model study. Our model results for pre-OAE2 and OAE2 conditions are compared to sediment records of organic carbon and proxies for photic zone euxinia and bottom water redox conditions (e. g. isorenieratane, carbon/phosphorus ratios). Our results show that a strongly elevated input of phosphorus from rivers and the Pacific Ocean relative to pre-OAE2 conditions is a requirement for the widespread development of low oxygen in the proto-North Atlantic during OAE2. Moreover, anoxia in the proto-North Atlantic is shown to be greatly influenced by the oxygen concentration of Pacific bottom waters. In our model, primary productivity increased significantly upon the transition from pre-OAE2 to OAE2 conditions. Our model captures the regional trends in anoxia as deduced from observations, with euxinia spreading to the northern and eastern shelves but with the most intense euxinia occurring along the southern coast. However, anoxia in the central deep basin is difficult to achieve in the model. This suggests that the ocean circulation used in the model may be too vigorous and/or that anoxia in the proto-North Atlantic was less widespread than previously thought

Reassessing the nitrogen isotope composition of sediments from the proto-North Atlantic during Oceanic Anoxic Event 2

Sediment records of the stable isotopic composition of N (δ15N) show light 15N values at several sites in the proto-North Atlantic during Oceanic Anoxic Event 2 (OAE 2) at the Cenomanian-Turonian transition (~94 Ma). The low δ15N during the event is generally attributed to anincrease in N2-fixation and incomplete uptake of NH+4 for phytoplankton growth. Surprisingly, published δ15N values for OAE 2 vary widely, even for similar locations. Using analyses of δ15N for sediments from three open-ocean and two coastal sites, we suggest that this reported variation is likely related to the treatment of sediment samples with acid prior to the δ15N analysis. A compilation of all available data for unacidifed samples for the proto North-Atlantic during OAE 2 demonstrates that the most pronounced negative shift in δ15N from pre-OAE 2 to OAE 2 occurs in the open ocean, but with δ15N never lower than -3‰. Using a box model of N cycling for the proto-North Atlantic during OAE 2, we show that N2-fixation is a major contributor to the δ15N signal, especially in the open ocean. Incomplete uptake of NH+4 for phytoplankton growth is important in regions dominated by downwelling, with lateral transport of NH+4 acting as a major source. In the southern proto-North Atlantic, where bottom waters were euxinic, the light δ15N signature is largely explained by upwelling of NH+4. Our study provides an overview of regional diferences in δ15N in the proto-North Atlantic and highlightsthe role of upwelling and high lateral exchange of water and nutrients, in addition to localbiogeochemical processes, in determining δ15N values of OAE 2 sediments

Trace metals as a redox proxy in Arabian Sea sediments in and below the oxygen minimum zone

Sedimentary trace metals are widely used to reconstruct past bottom water redox conditions. The calibration of trace metals as a redox proxy for oxygen minimum zones (OMZs) can still be improved. Here, we combine pore water and solid phase Mo, U, Re and V profiles with Fe and Mn data for tensites along a bottom water oxygen (2 to 83 μmol L−1 O 2) and water depth gradient (885 to 3010 m) in and below the perennial OMZ in the northern Arabian Sea (Murray Ridge). Trends in sedimentary Mo, U and Re contents generally follow ambient bottom water redox conditions, with the highest enrichments in OMZ sediments, supporting the validity of these trace metals as redox proxies. Vanadium and Fe are exclusively enriched in the sediments of the most anoxic OMZ site and do not capture further redox changes in and below the OMZ. We attribute the absence of a redox trend in sedimentary Fe content to the mildly reducing conditions in the sediments, with little FeS formation and benthic release of Fe. Manganese, in contrast, is depleted in the OMZ sediments and enriched in sediments below the OMZ, in accordance with loss from OMZ sediments and transfer of Mn to deeper sites (“Mn shuttling”). Manganese oxides are likely a key carrier of Mo and V to the sediments, especially below the OMZ, while diffusion across the sediment-water interface supplies U. Sedimentary Mo, U and V contents in the present-day Arabian Sea OMZ are generally lower than observed in other perennial OMZs. This may be related to a lower input of organic matter in this part of the Arabian Sea when compared to other OMZs, and hence, less anaerobic degradation of organic matter and less authigenic fixation of metals, even at the same bottom water oxygen concentrations. Our results have implications for the detection of OMZs in the geological record, implying that thresholds in trace metal concentrations for perennial OMZs may be lower than previously considered. Comparison of our Mo, U, Re and V data to trace metal records from 15 to 200 ka for the same Arabian Sea region suggests that the OMZ was periodically wider and more reducing in the past</p

Trace metals as a redox proxy in Arabian Sea sediments in and below the oxygen minimum zone

Sedimentary trace metals are widely used to reconstruct past bottom water redox conditions. The calibration of trace metals as a redox proxy for oxygen minimum zones (OMZs) can still be improved. Here, we combine pore water and solid phase Mo, U, Re and V profiles with Fe and Mn data for tensites along a bottom water oxygen (2 to 83 μmol L−1 O 2) and water depth gradient (885 to 3010 m) in and below the perennial OMZ in the northern Arabian Sea (Murray Ridge). Trends in sedimentary Mo, U and Re contents generally follow ambient bottom water redox conditions, with the highest enrichments in OMZ sediments, supporting the validity of these trace metals as redox proxies. Vanadium and Fe are exclusively enriched in the sediments of the most anoxic OMZ site and do not capture further redox changes in and below the OMZ. We attribute the absence of a redox trend in sedimentary Fe content to the mildly reducing conditions in the sediments, with little FeS formation and benthic release of Fe. Manganese, in contrast, is depleted in the OMZ sediments and enriched in sediments below the OMZ, in accordance with loss from OMZ sediments and transfer of Mn to deeper sites (“Mn shuttling”). Manganese oxides are likely a key carrier of Mo and V to the sediments, especially below the OMZ, while diffusion across the sediment-water interface supplies U. Sedimentary Mo, U and V contents in the present-day Arabian Sea OMZ are generally lower than observed in other perennial OMZs. This may be related to a lower input of organic matter in this part of the Arabian Sea when compared to other OMZs, and hence, less anaerobic degradation of organic matter and less authigenic fixation of metals, even at the same bottom water oxygen concentrations. Our results have implications for the detection of OMZs in the geological record, implying that thresholds in trace metal concentrations for perennial OMZs may be lower than previously considered. Comparison of our Mo, U, Re and V data to trace metal records from 15 to 200 ka for the same Arabian Sea region suggests that the OMZ was periodically wider and more reducing in the past</p

Links between temperature changes and oceanic-plateau emplacement during the Cenomanian–Turonian Oceanic Anoxic Event (OAE 2)

The Cenomanian–Turonian boundary interval (~94 Ma) was marked by a period of climatic turbulence, and featured the widespread expansion of strongly oxygen-depleted conditions across a large part of the global ocean; collectively these environmental degradations are referred to as an oceanic anoxic event or OAE (specifically OAE 2 for this time interval). Extremely high sea-surface temperatures are documented for several regions during OAE 2, likely beginning at the onset of the event, but a shift towards colder conditions during the early stages of the OAE (the Plenus Cold Event or PCE) is also recorded in several locales, before a return to a very warm climate during the latter part of the crisis. The overarching high temperatures are thought to have resulted from major volcanic activity during the emplacement of one or more oceanic plateaus, as evidenced by a globally documented shift in osmium-isotope ratios to very unradiogenic values just below the base of OAE strata that indicates a very large flux of mantle-like osmium to the open ocean at that time. Intriguingly, the PCE cooling has been shown as likely non-synchronous globally, suggesting a local control in addition to/instead of global forcing; whilst the high temperatures associated with OAE 2 appear to have continued long after the OAE itself ceased. This study presents new osmium-isotope data from the New Jersey shelf of the proto-North Atlantic (ODP Leg 174AX: Bass River), correlating the results with a previously generated sea-surface temperature dataset from the same site. These results are then compared with other temperature archives and osmium records of oceanic-plateau activity for OAE 2. The new data indicate intense oceanic-plateau activity prior to and in the earliest stages of the OAE, with a decline in mantle-osmium output before the end of the event, consistent with previous findings. However, when the osmium data are directly correlated with temperature records, both at Bass River and other sites, they clearly show that not only were high sea-surface temperatures maintained after the OAE, but also after oceanic-plateau activity (and presumably associated volcanism and CO2 emissions) fell. Thus, a reduction in mantle carbon output did not manifestly result in an immediate reduction of atmospheric CO2. Moreover, the beginning of the osmium recovery broadly correlates with the end of the PCE cooling at all locations where both osmium and temperature trends have been studied. Consequently, although the PCE cooling was not globally synchronous and its precise timing at individual locations was likely controlled by local processes, some feature of the oceanic-plateau development allowed the cooling spells to occur when plateau activity was most intense, before a reduction in that intensity stymied the spread of cold conditions and resulted in the restoration of high temperatures in the latter stages of the OAE and beyond. These data highlight the need for further work to understand the complexity of and nuances in the relationships between large-scale volcanism and major climate/environment perturbations, both for OAE 2 and for other events