Changes In Global Ocean Circulation Research Articles

Episodic intensification of marine phosphorus burial over the last 80 million years

Marine phosphatization events cause episodic carbonate fluorapatite (CFA) precipitation on seamounts, and are commonly linked to growth hiatuses in ferromanganese (Fe-Mn) crusts. However, the complete record of these events and their paleoenvironmental significance remains poorly understood, in large part due to poor age constraints. Here, we apply U-Pb dating to CFA in Fe-Mn crusts from Western Pacific seamounts. These data exhibit good alignment with Sr isotope ages, revealing six potential phosphatization events. This established CFA chronology tightens the timespan of phosphatization events and refines the age framework of Fe-Mn crusts. We subsequently utilize a multiproxy approach to demonstrate that the phosphatization events occurred coeval with the expansion of oceanic oxygen minimum zones. The Western Pacific Fe-Mn crusts thus document major perturbations in global oceanic phosphorus cycling, which appear to have been driven by climate-induced increases in primary productivity linked to changes in global ocean circulation.

2023 Record marine heat waves: coral reef bleaching HotSpot maps reveal global sea surface temperature extremes, coral mortality, and ocean circulation changes

Abstract Coral reefs, the most sensitive ecosystem to high temperature, are on the precipice of mass extinction from global warming [1, 2]. 2023 was the hottest year in recorded history on land and in the sea, with dramatic and unexpected temperature increases [3, 4]. Coral Reef Bleaching HotSpot maps provide unique insight into global ocean circulation changes in response to greenhouse gas (GHG) forcing that caused dramatic global temperature rises [1, 2]. The highest excess daily air temperatures recorded in 175 countries, as well as the most prolonged excessive sea surface temperatures, were centered around Jamaica. 2023 marked the worst coral reef bleaching yet in the Northern Hemisphere, with the Southern Hemisphere poised to follow in early 2024. The HotSpot maps strongly suggest accelerated ocean poleward heat transport, slowdown in upwelling, and decreased deep water formation linked to sharply increased 2023 anomalous sea surface and air temperatures. The 2023 distribution of severe heat and bleaching follows both spatial patterns and temporal trends first shown from a baseline 1982–2001 global SST trend analysis [5]. Increased warming of both hot and cold ocean currents shows that horizontal mixing of tropical heat to the poles is accelerating, and that vertical mixing with cold deep water is slowing down, leading to increased ocean stratification, which will cause sea temperature to increase more rapidly and CO2 mixing with the deep ocean to decrease.

Changes in Global Ocean Circulation due to Isopycnal Diffusion

Abstract We investigate changes in the ocean circulation due to the variation of isopycnal diffusivity (κiso) in a global non-eddy-resolving model. Although isopycnal diffusion is thought to have minor effects on interior density gradients, the model circulation shows a surprisingly large sensitivity to the changes: with increasing κiso, the strength of the Atlantic residual overturning circulation (AMOC) and the Antarctic Circumpolar Current (ACC) transport weaken. At high latitudes, the isopycnal diffusion diffuses temperature and salinity upward and poleward, and at low latitudes downward close to the surface. Increasing isopycnal diffusivity increases the meridional isopycnal fluxes whose meridional gradient is equatorward, hence leading to a negative contribution to the flux divergence in the tracer equations and predominant cooling and freshening equatorward of 40°. The effect on temperature overcompensates the countering effect of salinity diffusion, such that the meridional density differences decrease, along with which ACC and AMOC decrease. We diagnose the adjustment process to the new equilibrium with increased isopycnal diffusion to assess how the other terms in the tracer equations react to the increased κiso. It reveals that around ±40° latitude, the cooling induced by the increased isopycnal flux is only partly compensated by warming by advection, explaining the net cooling. Overall, the results emphasize the importance of isopycnal diffusion on ocean circulation and dynamics, and hence the necessity of its careful representation in models. Significance Statement The effect of mixing by mesoscale eddies, represented as diffusion along surfaces of constant density in models, on the ocean circulation is not well understood. Here, we show that an increase in the eddy diffusivity in different setups of a global ocean model leads to a surprisingly large change of the ocean circulation. The strength of the Atlantic overturning circulation and the Antarctic Circumpolar Current decrease. We find that the interior ocean becomes cooler and fresher and that the temperature effect on density dominates over salinity, resulting in a decrease in the density gradients. Our results point out the importance of eddy diffusion on ocean circulation, and hence the necessity of its correct representation in ocean and climate models.

The middle–late Tournaisian crisis in conodont diversity: a comparison between Northeast Laurussia and Northeast Siberia

The middle–late Tournaisian (Hastarian–Ivorian) transition is marked by isotopic and sedimentological evidence of climatic cooling and glaciations accompanied by a sea level fall and changes in global ocean circulation. At this time, the extinction among conodonts was followed by its gradual recovery in the late Tournaisian–early Viséan. Siphonodellids, representing an important part of conodont assemblages in the early–middle Tournaisian, became extinct. This study aims to compare conodont diversity dynamics in Northeast Laurussia and Northeast Siberia. The materials used range in paleolatitudinal gradients from 20°N (the north of the Urals and Pechora Craton) up to 45°N (Northeast Siberia), and in paleoclimatic gradient from an equatorial to a warm subtropical climate. The middle and late Tournaisian conodont associations of these regions demonstrate a high similarity in taxonomic composition and diversity dynamics, which may suggest that the glaciation and the following decrease in temperature probably was not the direct cause of conodont extinction; instead, the changes in the food web implied by the carbon isotope compositions of conodont elements and host carbonates could have been the main driver of the crisis in conodont diversity

The influence of variability in meridional over turning on global ocean circulation

AbstractThe Atlantic Meridional Overturning circulation (AMOC) is an important global scale circulation and changes in AMOC can induce significant regional and global climate impacts. Here we study the stability of AMOC and its influence on global ocean circulation and the surface climate though analyzing a set of sensitivity experiments using the Community Climate System Model version 1 (CESM1). Results show that a collapsed AMOC can induce changes in global ocean circulation, such as reduced (or reversed) Bering Strait transport, weakened Indonesia throughflow and Agulhas current, but strengthened Drake Passage transport. It also changes the global wind pattern and surface temperature, such as a seesaw-like surface temperature change between Northern and Southern Hemispheres, a weakening of Indian-Australia summer monsoon, a southward shift of the Southern ocean westerlies, etc. We also found that AMOC and the Pacific deep meridional overturning circulation (or PMOC) do not form a natural seesaw under modern day climate and geography. A collapsed AMOC (active PMOC) is not stable under modern conditions if there is no additional freshwater (salt) input in the subpolar North Atlantic (Pacific), suggesting that the modern mean state of AMOC (PMOC) does not depend on local haline forcing although its variability and changes do.

Lead isotopes in deep-sea coral skeletons: Ground-truthing and a first deglacial Southern Ocean record

Past changes in seawater lead (Pb) isotopes record the temporal evolution of anthropogenic pollution, continental weathering inputs, and ocean current transport. To advance our ability to reconstruct this signature, we present methodological developments that allow us to make precise and accurate Pb isotope measurements on deep-sea coral aragonite, and apply our approach to generate the first Pb isotope record for the glacial to deglacial mid-depth Southern Ocean.Our refined methodology includes a two-step anion exchange chemistry procedure and measurement using a 207Pb–204Pb double spike on a Thermo Finnigan Triton TIMS instrument. By employing a 1012Ω resistor (in place of a 1011Ω resistor) to measure the low-abundance 204Pb ion beam, we improve the internal precision on 206,207,208Pb/204Pb for a 2ng load of NIST-SRM-981 Pb from typically ∼420ppm to ∼230ppm (2 s.e.), and the long term external reproducibility from ∼950ppm to ∼550ppm (2 s.d.). Furthermore, for a typical 500mg coral sample with low Pb concentrations (∼6–10ppb yielding ∼3–5ngPb for analysis), we obtain a comparable internal precision of ∼150–250ppm for 206,207,208Pb/204Pb, indicating a good sensitivity for tracing natural Pb sources to the oceans. Successful extraction of a seawater signal from deep-sea coral aragonite further relies on careful physical and chemical cleaning steps, which are necessary to remove anthropogenic Pb contaminants and obtain results that are consistent with ferromanganese crusts.Applying our approach to a collection of late glacial and deglacial corals (∼12–40kaBP) from south of Tasmania at ∼1.4–1.7km water depth, we generated the first intermediate water Pb isotope record from the Southern Ocean. That record reveals millennial timescale variability, controlled by binary mixing between two Pb sources, but no distinct glacial-interglacial Pb isotope shift. Mixing between natural endmembers is fully consistent with our data and points to a persistence of the same Pb sources through time, although we cannot rule out a minor influence from recent anthropogenic Pb. Whereas neodymium (Nd) isotopes in the Southern Ocean respond to global ocean circulation changes between glacial and interglacial periods, Pb isotopes record more localised mixing within the Antarctic Circumpolar Current, potentially further modulated by climate through changing terrestrial inputs from southern Africa or Australia. Such decoupling between Pb and Nd isotopes in the Southern Ocean highlights their potential to provide complementary insights into past oceanographic variability.Keywords: Southern Ocean; Tasmania; Deglaciation; Climate change; Ocean circulation; Weathering; Isotope tracers; Pb isotopes; Deep-sea corals

Onset of Antarctic Circumpolar Current 30 million years ago as Tasmanian Gateway aligned with westerlies.

Earth's mightiest ocean current, the Antarctic Circumpolar Current (ACC), regulates the exchange of heat and carbon between the ocean and the atmosphere, and influences vertical ocean structure, deep-water production and the global distribution of nutrients and chemical tracers. The eastward-flowing ACC occupies a unique circumglobal pathway in the Southern Ocean that was enabled by the tectonic opening of key oceanic gateways during the break-up of Gondwana (for example, by the opening of the Tasmanian Gateway, which connects the Indian and Pacific oceans). Although the ACC is a key component of Earth's present and past climate system, the timing of the appearance of diagnostic features of the ACC (for example, low zonal gradients in water-mass tracer fields) is poorly known and represents a fundamental gap in our understanding of Earth history. Here we show, using geophysically determined positions of continent-ocean boundaries, that the deep Tasmanian Gateway opened 33.5 ± 1.5 million years ago (the errors indicate uncertainty in the boundary positions). Following this opening, sediments from Indian and Pacific cores recorded Pacific-type neodymium isotope ratios, revealing deep westward flow equivalent to the present-day Antarctic Slope Current. We observe onset of the ACC at around 30 million years ago, when Southern Ocean neodymium isotopes record a permanent shift to modern Indian-Atlantic ratios. Our reconstructions of ocean circulation show that massive reorganization and homogenization of Southern Ocean water masses coincided with migration of the northern margin of the Tasmanian Gateway into the mid-latitude westerly wind band, which we reconstruct at 64° S, near to the northern margin. Onset of the ACC about 30 million years ago coincided with major changes in global ocean circulation and probably contributed to the lower atmospheric carbon dioxide levels that appear after this time.

The TICE event: Perturbation of carbon–nitrogen cycles during the mid-Tournaisian (Early Carboniferous) greenhouse–icehouse transition

Carbonate carbon (δ13Ccarb) and bulk nitrogen (δ15N) isotopic variation during the mid-Tournaisian was analyzed in the Malanbian and Long'an sections of South China. Their C-isotope profiles document a large positive excursion, herein termed the ‘mid-Tournaisian carbon isotope excursion’ (TICE), during the Siphonodella isosticha conodont Zone, although its magnitude differed between the two sections (>6‰ at Malanbian versus ~3‰ at Long'an). The TICE event coincided with sedimentologic and oxygen-isotopic evidence of climatic cooling and glaciation during the mid-Tournaisian. It was probably triggered by an increase in organic carbon burial rates linked to changes in global-ocean circulation. The study sections also document a large positive shift in δ15N, from 1.7‰ to 4.2‰ at Malanbian and from 1.5‰ to 3.8‰ at Long'an. The N-isotope shift shows no termination within the study sections and is likely to mark the onset of an extended interval of 15N-enriched marine nitrate that lasted for the duration of the Late Paleozoic Ice Age. Its initiation coincided with TICE and thus may have been linked to ocean-circulation changes that resulted in intensified upwelling and an increase in water-column denitrification. The continuation of the N-isotope shift over millions of years may have been linked to glacio-eustatic fall and a long-term shift in the locus of denitrification from continental-shelf sediments to continent-margin oxygen-minimum zones. The TICE event thus marks the onset of sustained continental glaciation during the Late Paleozoic Ice Age.

Persistently strong Indonesian Throughflow during marine isotope stage 3: evidence from radiogenic isotopes

The Indonesian Throughflow (ITF) connects the western Pacific Ocean with the eastern Indian Ocean, thus forming one of the major near surface current systems of the global thermohaline circulation. The intensity of the ITF has been found to be sensitive to changes in global ocean circulation, fluctuations in sea level, as well as to the prevailing monsoonal conditions of the Indonesian Archipelago and NW Australia. This study presents the first reconstruction of ITF dynamics combining radiogenic isotope compositions of neodymium (Nd), strontium (Sr), and lead (Pb) of the clay-size detrital fraction to investigate changes in sediment provenance, and paleo seawater Nd signatures extracted from the planktonic foraminifera and authigenic Fe–Mn oxyhydroxide coatings of the marine sediments focussing on marine isotope stage 3 (MIS3). Sediment core MD01-2378 was recovered within the framework of the International Marine Global Change Study (IMAGES) and is located in the area of the ITF outflow in the western Timor Sea (Scott Plateau, 13° 04.95′ S and 121° 47.27′ E, 1783 m water depth). In order to produce reliable seawater signatures, several extraction methods were tested against each other. The results of the study show that at this core location the extraction of surface water Nd isotope compositions from planktonic foraminifera is complicated by incomplete removal of contributions from Fe–Mn oxyhydroxides carrying ambient bottom water signatures. The bottom water Nd isotope signatures reliably obtained from the sediment coatings (average εNd = −5.0) document an essentially invariable water mass composition similar to today throughout the entire MIS3. The radiogenic Nd, Sr, and Pb isotope records of the clay-sized detrital fraction suggest that the Indonesian Archipelago rather than NW Australia was the main particle source at the location of core MD01-2378, and thus indicating a persistently strong ITF during MIS3. Furthermore, the variations of the detrital radiogenic isotopes are shown to be more sensitive to changes in circulation and document a somewhat enhanced ITF intensity during the early part of MIS3 until 47.4ka compared with the remaining MIS3.

Millennial‐scale glacial meltwater pulses and their effect on the spatiotemporal benthicδ18O variability

Ratios of oxygen isotope values obtained from foraminiferal calcite are one of the most established paleoceanographic proxies. They are used in the context of estimating variations in ice volume, ocean temperature, and salinity and provide a means to date marine sediment cores across different ocean basins. Our study addresses the question how the δ18O of the deglacial meltwater signal propagates into the interior ocean, when large‐scale millennial‐scale reorganizations of the Atlantic Meridional Overturning Circulation (AMOC) are present. Analyzing a series of idealized tracer‐injection experiments conducted with an earth system model, we find that a substantial weakening of the AMOC leads to a massive delay in the export of the glacioeustatic oxygen isotope signal into the deep ocean, whereas the Atlantic‐Pacific lag in benthic oxygen isotope signals is not increased. Furthermore, it is shown that an AMOC cessation causes a decoupling ofδ18O propagation time and water mass age, in particular in the deep Pacific. Our results lend further support to the notion that benthic oxygen isotope records obtained from stacks are not a useful global chronostratigraphic tool during periods of millennial‐scale global ocean circulation changes. The regionally varying delay effect of the deglacial sea level signal studied here adds onto existing uncertainties in the interpretation and decomposition of benthic oxygen isotope in terms of sea level, temperature, and hydrographic variations.

Hypothesis for mega-outburst flooding from a palaeo-subglacial lake beneath the East Antarctic Ice Sheet

Subglacial outburst floods at the margins of the Antarctic and Laurentide Ice Sheets have been linked to changes in global ocean circulation and climate. The impact of palaeo-hydrological systems beneath the East Antarctic Ice Sheet (EAIS) has, however, remained elusive. By analysing bed morphology in the Wilkes Subglacial Basin, we hypothesise the occurrence of a major palaeo-subglacial lake and associated outburst floods in the interior of East Antarctica. At similar to 70 km wide and over 100 km long, the inferred area of outburst flooding is the most extensive in Antarctica, and the palaeo-subglacial lake from which the floods originated is the second largest after Lake Vostok. The scale of inferred outburst flooding is similar to mega-floods beneath the Laurentide Ice Sheet. We suggest that this major hydrological system developed during expansion of the EAIS in the middle Miocene and probably affected ice sheet stability, ocean circulation and climate evolution.

Reducing Vulnerability of Agriculture and Forestry to Climate Variability and Change: Workshop Summary and Recommendations

The International Workshop on Reducing Vulnerability of Agriculture and Forestry to Climate Variability and Climate Change held in Ljubljana, Solvenia, from 7 to 9 October 2002 addressed a range of important issues relating to climate variability, climate change, agriculture, and forestry including the state of agriculture and forestry and agrometeological information, and potential adaptation strategies for agriculture and forestry to changing climate conditions and other pressures. There is evidence that global warming over the last millennium has already resulted in increased global average annual temperature and changes in rainfall, with the 1990s being likely the warmest decade in the Northern Hemisphere at least. During the past century, changes in temperature patterns have, for example, had a direct impact on the number of frost days and the length of growing seasons with significant implications for agriculture and forestry. Land cover changes, changes in global ocean circulation and sea surface temperature patterns, and changes in the composition of the global atmosphere are leading to changes in rainfall. These changes may be more pronounced in the tropics. For example, crop varieties grown in the Sahel may not be able to withstand the projected warming trends and will certainly be at risk due to projected lower amounts of rainfall as well. Seasonal to interannual climate forecasts will definitely improve in the future with a better understanding of dynamic relationships. However, the main issue at present is how to make better use of the existing information and dispersion of knowledge to the farm level. Direct participation by the farming communities in pilot projects on agrometeorological services will be essential to determine the actual value of forecasts and to better identify the specific user needs. Old (visits, extension radio) and new (internet) communication techniques, when adapted to local applications, may assist in the dissemination of useful information to the farmers and decision makers. Some farming systems with an inherent resilience may adapt more readily to climate pressures, making long-term adjustments to varying and changing conditions. Other systems will need interventions for adaptation that should be more strongly supported by agrometeorological services for agricultural producers. This applies, among others, to systems where pests and diseases play an important role. Scientists have to guide policy makers in fostering an environment in which adaptation strategies can be effected. There is a clear need for integrating preparedness for climate variability and climate change. In developed countries, a trend of higher yields, but with greater annual fluctuations and changes in cropping patterns and crop calendars can be expected with changing climate scenarios. Shifts in projected cropping patterns can be disruptive to rural societies in general. However, developed countries have the technology to adapt more readily to the projected climate changes. In many developing countries, the present conditions of agriculture and forestry are already marginal, due to degradation of natural resources, the use of inappropriate technologies and other stresses. For these reasons, the ability to adapt will be more difficult in the tropics and subtropics and in countries in transition. Food security will remain a problem in many developing countries. Nevertheless, there are many examples of traditional knowledge, indigenous technologies and local innovations that can be used effectively as a foundation for improved farming systems. Before developing adaptation strategies, it is essential to learn from the actual difficulties faced by farmers to cope with risk management at the farm level. Agrometeorologists must play an important role in assisting farmers with the development of feasible strategies to adapt to climate variability and climate change. Agrometeorologists should also advise national policy makers on the urgent need to cope with the vulnerabilities of agriculture and forestry to climate variability and climate change. The workshop recommendations were largely limited to adaptation. Adaptation to the adverse effects of climate variability and climate change is of high priority for nearly all countries, but developing countries are particularly vulnerable. Effective measures to cope with vulnerability and adaptation need to be developed at all levels. Capacity building must be integrated into adaptation measures for sustainable agricultural development strategies. Consequently, nations must develop strategies that effectively focus on specific regional issues to promote sustainable development.

The Southern Ocean surface between Marine Isotope Stages 6 and 5d: Shape and timing of climate changes

Summer sea surface temperature (SSST) and sea-ice extent, obtained at millennial to centennial resolution, are presented for seven cores recovered in the Atlantic and western Indian Antarctic Zone, from Termination II and the last interglacial s.s., i.e. Marine Isotope Substage (MIS) 5e. The diatom-based SSST estimates show that during the late MIS 6 intensified export of cold waters from the Weddell Sea resulted in SSSTs close to 0°C in the central and eastern Atlantic sector. Due to northward expansion of the winter sea-ice edge by 3–5° latitude the sea-ice seasonality was reduced. Such conditions have great potential for efficient year-round reduction in air–sea gas exchange and glacial CO 2 draw down. The warming at Termination II started at 132–131 ka, marked by rapid retreat of the sea-ice edge. The SSST increased within a few thousands of years to reach maximum interglacial summer temperatures ranging between 2.5 and 3.5°C, in the earliest MIS 5e (128–125 ka). Besides a distinct cold reversal around 129.5 ka, the warming was punctuated by short-lived temperature rebounds with amplitudes of 1–1.5°C, occurring at 200–300-yr frequency, thus in the range of the periodicity of solar input. Increased Chaetoceros spp. abundance suggests meltwater events as the primary cause of the coolings. The SSST optimum, characterised by strongly enhanced biogenic sedimentation rates, ended at ca. 125 ka, with the full re-establishment of the global conveyor belt and beginning of the Northern Hemisphere climatic optimum. At this time SSSTs decreased stepwise and stabilised on values slightly warmer than today, showing in the following period of MIS 5e only low amplitude oscillations. During MIS 5d a northward expansion of the sea-ice field took place, but SSSTs only slightly colder than modern ones and a weakened meridional temperature gradient indicate that the Southern Ocean did not fall in a true glacial mode. The tight correlation between Southern Ocean SSST and winter sea-ice records, and Vostok atmospheric temperature and CO 2 records support the idea that the Antarctic winter sea-ice and Southern Ocean temperature exert a primary control on the atmospheric–ocean CO 2 exchanges and hence on global climate. Reduction of the Antarctic sea-ice field and increase of SSST at the onset of Termination II could have been triggered by precessional changes influencing southern high latitude summer insolation. Positive feedback mechanisms, reduction of surface albedo and release of CO 2 into the atmosphere, and changes in global ocean circulation, affected by a collapse or strong reduction in North Atlantic Deep Water (NADW) production, could have induced further reinforcement of the Southern Ocean warming.

Early Oligocene initiation of North Atlantic Deep Water formation.

Dating the onset of deep-water flow between the Arctic and North Atlantic oceans is critical for modelling climate change in the Northern Hemisphere and for explaining changes in global ocean circulation throughout the Cenozoic era (from about 65 million years ago to the present). In the early Cenozoic era, exchange between these two ocean basins was inhibited by the Greenland-Scotland ridge, but a gateway through the Faeroe-Shetland basin has been hypothesized. Previous estimates of the date marking the onset of deep-water circulation through this basin-on the basis of circumstantial evidence from neighbouring basins-have been contradictory, ranging from about 35 to 15 million years ago. Here we describe the newly discovered Southeast Faeroes drift, which extends for 120 km parallel to the basin axis. The onset of deposition in this drift has been dated to the early Oligocene epoch ( approximately 35 million years ago) from a petroleum exploration borehole. We show that the drift was deposited under a southerly flow regime, and conclude that the initiation of deep-water circulation from the Norwegian Sea into the North Atlantic Ocean took place much earlier than is currently assumed in most numerical models of ancient ocean circulation.

後期第四紀における基礎生産力の推定法

It has been suggested that the observed late Quaternay fluctuation in atmospheric CO2 concentrations is a function of temporal changes in global ocean circulation and the level of primary productivity in the ocean. Therefore the estimation of palaeo-productivity offers clues to understand the large fluctuation of atmospheric CO2 in association with the climatic change and to predict future changes in the greenhouse effects which result from the contribution of anthropogenic CO2 to natural carbon cycle in the earth surface. In this paper, the various reconstruction methods for the estimation of palaeo-productivity are reviewed. Quantitatively estimation has been proposed, using equations based upon organic carbon accumulation rates and assemblages of foraminifers and diatoms. On the other hand, qualitative estimates have been made, using accumulation rates of biogenic opal, inorganic elements and biogenic fossils. Although various reconstruction methods provide a relatively good signal of productivity changes, the further studies on degradation of sinking particles within the water column and on the ealy diagenesis in the surface sediments are required in order to produce productivity estimates with more precise productivity oscillations and a more absolute productivity level.