While several hypotheses exist to explain the development of large-scale perennial Northern Hemisphere ice sheets in the late Pliocene and early Pleistocene, the prevailing view is that a decline in atmospheric carbon dioxide (CO2) drove this substantial change in late Neogene climate. However, the primary mechanism responsible for this reduction in CO2 has yet to be fully explored. Mineral dust-derived iron enhancement of ocean organic carbon production and export to the deep ocean and marine sediments has previously been invoked to explain reductions in atmospheric CO2 on multiple timescales. Here we test the hypothesis that iron fertilization of the Pliocene subarctic North Pacific affected atmospheric CO2, and in turn drove the formation of Northern Hemisphere ice sheets. By compiling Pliocene dust and export productivity proxy data sets from across the North Pacific and then progressively filtering for the most reliable records, we find that there is no relationship between North Pacific dust inputs and export production in the Pliocene. Finally, we apply these new composites to broadly assess previously proposed drivers of Pliocene Asian dust dynamics as well as North Pacific Ocean circulation and biogeochemistry.

Marine sedimentary beryllium-10 (Be-10) has been used to constrain many paleoenvironmental processes over the past 10 million years, but dynamic processes such as particle scavenging and ocean circulation can complicate interpretation of sedimentary Be-10 data. We generated six new sedimentary Be-10 flux records, normalized using excess thorium-230 (Th-230(XS)), and present a global compilation of Th-230(XS)-normalized Be-10 flux data combined with Th-230(XS)-normalized bulk sediment flux and sediment composition data to improve constraints on the climatic and oceanographic factors governing Be-10 fluxes to the seafloor. Our analysis suggests that the first order factors driving global variability in sedimentary Be-10 fluxes are (1) bulk sediment flux; (2) sediment composition; and (3) a combination of ocean basin and sediment core water depth that likely reflects the overlying water mass concentration of Be-10. While these parameters explain similar to 60-80% of the variance observed in the core top and Holocene data, the correlations exhibited between these parameters and Be-10 fluxes are much weaker in the 0-250 ka time series data. The increased variability observed in the time series data likely results from climatically-driven changes in the available seawater Be-10 inventory within each water mass over time, although we cannot rule out the impact of variable carbonate and opal dissolution. The sensitivity of abyssal sediment Be-10 fluxes to seawater Be-10 concentrations suggests that sedimentary Be-10 could be used to reconstruct past changes in basin-scale particle fluxes and ocean circulation, analogous to the shorter-lived protactinium-231 (Pa-231) and Th-230(XS) systems, throughout the Late Neogene and supports further refinement of Be-10 as a paleoceanographic proxy.

Antarctic ice core evidence indicates that atmospheric CO2 levels increased during Heinrich Stadial (HS) 1 and the Younger Dryas (YD) during the last deglaciation. A substantial fraction of this carbon is believed to have stemmed from the ocean interior, released, in part, through enhanced wind-driven upwelling and air-sea CO2 exchange in the Southern Ocean. This was highlighted by two deglacial opal flux peaks identified in sediment core TN057-13-PC4 (53.17 °S, 5.13 °E, 2818 m water depth) from the Atlantic Southern Ocean south of the Polar Front, proximal to the Antarctic Divergence Zone (Anderson et al., 2009). However, there is limited information on changes in deep-ocean 14C ventilation and surface ocean hydrography in the Atlantic Antarctic Divergence, and their role in atmospheric CO2 variations during these two periods of deglacial CO2 rise. Here, we provide a new set of 12 mixed-benthic and 63 planktonic foraminiferal (i.e., Neogloboquadrina pachyderma) 14C ages obtained with a MIni-CArbon-DAting-System (MICADAS) in sediment core TN057-13-PC4, along with high-resolution multi-proxy (sub-)sea surface temperature reconstructions for the same site (N. pachyderma Mg/Ca ratios, TEX86, diatom assemblages). Our data help better constrain the nature, timing, and impacts of deep-ocean upwelling on surface ocean hydrography and on atmospheric CO2 exchange near the Antarctic Divergence of the Southern Ocean. Our data show strong (sub-)surface warming in the Antarctic Divergence during HS1 and YD that is accompanied by a rapid decline in benthic-minus-planktic 14C ages towards mean Holocene values at the onset of the deglaciation. We also observe millennial-scale increases in seawater d18O (paired N. pachyderma Mg/Ca-d18O analyses), hence local surface salinity and marked variations in 14C surface ocean reservoir ages that parallel changes in Antarctic sea ice extent. This corroborates previous evidence indicating increased upwelling of Circumpolar Deep Water in the Atlantic Antarctic Divergence during HS1 and YD, yet suggests an onset of strong Southern Ocean ventilation earlier than what is expected from increases in opal fluxes alone. Our data support a fundamental role of upwelling and CO2 outgassing in the Antarctic Divergence of the Southern Ocean in the two-step atmospheric CO2 rise during the last deglaciation, and further suggest that possible variations in CO2 solubility and sea-ice retreat amplified the effects of physical circulation changes on Southern Ocean air-sea CO2 exchange.References: Anderson, R.F., Ali, S., Bradtmiller, L.I., Nielsen, S.H.H., Fleisher, M.Q., Anderson, B., Burckle, L.H., 2009. Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2. Science 323, 1443–1448. doi: 10.1126/science.116744

An important role in the cycling of marine trace elements is scavenging, their adsorption and removal from the water column by sinking particles. Boundary scavenging occurs when areas of strong particle flux drive preferential removal of the trace metals at locations of enhanced scavenging. Due to its uniform production and quick burial via scavenging, 230 Th is used to assess sedimentary mass fluxes; however, these calculations are potentially biased near regions where net lateral transport of dissolved 230 Th violates the assumption that the flux of particulate 230 Th to the seabed equals its rate of production in the water column. Here, we present a water column transect of dissolved 230 Th along 152° W between Alaska and Tahiti (GEOTRACES GP15), where we examine 230 Th profiles across multiple biogeochemical provinces and, novelly, the lateral transport of 230 Th to distal East Pacific Rise hydrothermal plumes. We observed a strong relationship between the slope of dissolved 230 Th concentration‐depth profiles and suspended particle matter inventory in the upper‐mid water column, reinforcing the view that biogenic particle mass flux sets the background 230 Th distribution in open ocean settings. We find that, instead of the region of enhanced particle flux around the equator, hydrothermal plumes act as a regional boundary sink of 230 Th. At 152° W, we found that the flux‐to‐production ratio, and thereby error in 230 Th‐normalized sediment flux, is between 0.80 and 1.50 for hydrothermal water, but the error is likely larger approaching the East Pacific Rise.

The physical and biogeochemical properties of the western Arctic Ocean are rapidly changing, resulting in cascading shifts to the local ecosystems. The nutrient‐rich Pacific water inflow to the Arctic through the Bering Strait is modified on the Chukchi and East Siberian shelves by brine rejection during sea ice formation, resulting in a strong halocline (called the Upper Halocline Layer (UHL)) that separates the cold and relatively fresh surface layer from the warmer and more saline (and nutrient‐poor) Atlantic‐derived water below. Biogeochemical signals entrained into the UHL result from Pacific Waters modified by sediment and river influence on the shelf. In this synthesis, we bring together data from the 2015 Arctic U.S. GEOTRACES program to implement a multi‐tracer (dissolved and particulate trace elements, radioactive and stable isotopes, macronutrients, and dissolved gas/atmospheric tracers) approach to assess the relative influence of shelf sediments, rivers, and Pacific seawater contribution to the Amerasian Arctic halocline. For each element, we characterized their behavior as mixing dominated (e.g., dCu, dGa), shelf‐influenced (e.g., dFe, dZn), or a combination of both (e.g., dBa, dNi). Leveraging this framework, we assessed sources and sinks contributing to elemental distributions: shelf sediments (e.g., dFe, dZn, dCd, dHg), riverine sources, (e.g., dCu, dBa, dissolved organic carbon), and scavenging by particles originating on the shelf (e.g., dFe, dMn, dV, etc.). Additionally, synthesized results from isotopic and atmospheric tracers yielded tracer age estimates for the Upper Halocline ranging between 1 and 2 decades on a spatial gradient consistent with cyclonic circulation.

Unsupported 231Pa/230Th ratios have been used widely as a paleoproxy for ocean circulation and as a paleoproductivity proxy; however, some of the inherent assumptions for these proxies have not been thoroughly tested, which would impact how the ratio is interpreted in different regions. Both applications of the ratio influence interpretations of past climate changes, so it is important to determine the extent to which each process impacts the sedimentary 231Pa/230Th values. Here, we compare 231Pa/230Th ratios between the Atlantic, Indian, and Pacific Oceans, and within the Atlantic Ocean, as a test of whether or not this ratio serves as a reliable proxy for water mass ventilation age, which is closely related to ocean circulation.In this study, we present new 231Pa and 230Th measurements from Indian and Atlantic Ocean sediments and from Indian Ocean seawater samples, alongside previously published Atlantic Ocean water column and sedimentary 231Pa/230Th ratios. The observed water column dissolved 231Pa/230Th profiles do not show the expected increase in ratio values with water mass age, which disagrees with the conceptual model for the use of 231Pa/230Th as a paleocirculation proxy. Dissolved 231Pa/230Th varies between 0.35 and 0.75 within the depth range of NADW, and there is no apparent correlation with water mass age. The observed 231Pa/230Th values and profiles are strikingly similar between the western North Atlantic, Western Indian, and North Pacific Oceans, even though these basins have significantly different water mass histories and deep water mass ages.The water column 231Pa/230Th values then determine the ratio value of the underlying sediments. So, we then compare 231Pa/230Th values from Holocene sediments between the ocean basins to determine if the observations agree with our understanding of modern ocean circulation. Atlantic and Indian Ocean sediments are indistinguishable from each other with respect to their 231Pa/230Th ratios. Taken together, these results indicate that factors other than ventilation age must significantly impact 231Pa/230Th ratios, and the evidence suggests that fractionation and scavenging intensity may strongly impact particulate and sedimentary 231Pa/230Th.

Quantitative records of bottom water oxygen (BWO) are critical for understanding deep ocean change through time. Because of the stoichiometric relationship between oxygen and carbon, BWO records provide insight into the physical and biogeochemical processes that control the air‐sea partitioning of both gases with important implications for climate over Quaternary glacial‐interglacial cycles. Here, we present new geochemical data sets from Ocean Discovery Program Site 1240 in the eastern equatorial Pacific to constrain paleoproductivity (Ba xs flux) and BWO using a multiproxy approach (aU, Mn/Al, Δδ 13 C, and U/Ba). This combination of approaches allows us to quantitatively identify changes in BWO and to parse local and basin‐wide contributions to the signal. We find that upwelling, not dust input, is responsible for driving productivity changes at the site. Changes in local carbon export are not the primary driver of changes in BWO, which instead reflect basin‐wide changes driven by processes in the Southern Ocean. Our BWO results provide direct evidence for the role of orbital precession and obliquity in driving deep sea respired carbon and oxygen concentrations. We find variations in BWO on the order of ∼50 μmol/kg that occur with ∼23 kyr periodicity during the substages of Marine Isotope Stage 5, and variations of ∼100 μmol/kg on glacial‐interglacial timescales. These findings have important implications for the role of insolation in driving deep ocean respired oxygen and carbon concentrations, and point to physical and biogeochemical changes in the Southern Ocean as key drivers of planetary‐scale carbon change.

Ice-core measurements show diverse atmospheric CO2 variations – increasing, decreasing or remaining stable – during millennial-scale North Atlantic cold periods called stadials. The reasons for these contrasting trends remain elusive. Ventilation of carbon-rich deep oceans can profoundly affect atmospheric CO2, but its millennial-scale history is poorly constrained. In this study, I will show a high-resolution deep-water acidity record from the Iberian Margin in the North Atlantic, a unique setting that allows us to construct a robust chronology for confident comparisons between marine and ice-core records. The new data combined with ice-core CO2 records reveal multiple ocean ventilation modes involving an interplay of the two polar regions, rather than by the Southern Ocean alone. These modes governed past deep-sea carbon storage and thereby atmospheric CO2 variations on millennial timescales. Overall, our record suggests a bipolar control on millennial atmospheric CO2 changes during the past glacial cycle.

A growing body of observations has revealed rapid changes in both the total inventory and the distribution of marine oxygen over the latter half of the 20th century, leading to increased interest in extending oxygenation records into the past. The use of paleo-oxygen proxies has the potential to extend the spatial and temporal range of current records, constrain pre-anthropogenic baselines, provide datasets necessary to test climate models under different boundary conditions, and ultimately understand how ocean oxygenation responds beyond decadal-scale changes. This review seeks to summarize the current state of knowledge about proxies for reconstructing Cenozoic marine oxygen: sedimentary features, sedimentary redox-sensitive trace elements and isotopes, biomarkers, nitrogen isotopes, foraminiferal trace elements, foraminiferal assemblages, foraminiferal morphometrics, and benthic foraminiferal carbon isotope gradients. Taking stock of each proxy reveals some common limitations as the majority of proxies functions best at low-oxygen concentrations, and many reflect multiple environmental drivers. We also highlight recent breakthroughs in geochemistry and proxy approaches to constraining pelagic (in addition to benthic) oxygenation that are rapidly advancing the field. In light of both the emergence of new proxies and the persistent multiple driver problem, the need for multi-proxy approaches and data storage and sharing that adhere to the principles of findability, accessibility, interoperability, and reusability (FAIR) is emphasized. Continued refinements of proxy approaches and both proxy–proxy and proxy–model comparisons are likely to support the growing needs of both oceanographers and paleoceanographers interested in paleo-oxygenation records.

Disentangling inputs of aeolian dust, ice-rafted debris (IRD), and eroded continental detritus delivered by ocean currents to marine sediments provide important insights into Earth System processes and climate. This study uses Sr-Nd-Pb isotope ratios of the continent-derived (lithogenic) fraction in deep-sea core TN057-6 from the subantarctic Southern Ocean southwest of Africa over the past 150,000 y to identify source regions and quantify their relative contributions and fluxes utilizing a mixing model set in a Bayesian framework. The data are compared with proxies from parallel core Ocean Drilling Program Site 1090 and newly presented data from potential South America aeolian dust source areas (PSAs), allowing for an integrated investigation into atmospheric, oceanic, and cryospheric dynamics. PSA inputs varied on glacial/interglacial timescales, with southern South American sources dominating up to 88% of the lithogenic fraction (mainly Patagonia, which provided up to 68%) during cold periods, while southern African sources were more important during interglacials. During the warmer Marine Isotope Stage (MIS) 3 of the last glacial period, lithogenic fluxes were twice that of colder MIS2 and MIS4 at times, and showed unique isotope ratios best explained by Antarctic-derived IRD, likely from the Weddell Sea. The IRD intrusions contributed up to 41% at times and followed Antarctic millennial warming events that raised temperatures, causing instability of icesheet margins. High IRD was synchronous with increased bioavailable iron, nutrient utilization, high biological productivity, and decreased atmospheric CO 2 . Overall, TN057-6 sediments record systematic Southern Hemisphere climate shifts and cryospheric changes that impacted biogeochemical cycling on both glacial/interglacial and subglacial timescales.

GEOTRACES is an international program that has benefited from contributions by investigators in 35 nations. The program mission is to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean and to establish the sensitivity of these distributions to changing environmental conditions. This perspective first summarizes the historical motivation for the program, and then describes selected research highlights, focusing on recent findings related to iron. The patchy distribution of iron in the ocean indicates a short residence time, at the low end of the range of residence times estimated in models. Iron removal from the ocean must, therefore, be rapid. Recent results from the North Atlantic Ocean suggest that the formation of particulate authigenic iron phases may be a factor contributing to iron removal that is faster than previously thought. This article also identifies several areas where advancements are expected through modeling and synthesis efforts.

A common technique for determining the mass accumulation rates of marine sediments is through measurements of constant flux proxies. These proxies, like unsupported 230Th and extraterrestrial 3He, are buried at a known rate at the seafloor, and thus their concentration in sediments is inversely proportional to the vertical mass accumulation rate of ambient sediments. These two proxies have varied assumptions regarding their behavior that have been difficult to test, particularly in regions of very low sedimentation. We present new measurements of helium and thorium isotopes in coretop sediments from the South Pacific Gyre. Our main finding is that 230Th- and 3He-derived mass accumulation rates of these sediments differ by a factor of 2-10, with 230Th-based mass accumulation rates systematically higher than those derived from 3He. While the final cause of the discrepancy is difficult to elucidate, we suggest that up to a factor of 2 worth of the sedimentary 230Th deficit can be explained by lateral transport of 230Th, while the remainder must be explained by other processes, such as ancient coretops undercorrected for post-depositional 230Th decay. Regardless of the mechanism, our findings have critical implications for the application of constant flux proxies in the South Pacific Gyre, with one example being the calibration of dust deposition models in this region. Ultimately, additional testing is required to determine the accuracy of the 3He and 230Th techniques for estimating sediment accumulation rates in regions of the open ocean with extremely low sedimentation rates.

Uranium is a redox-sensitive trace element that under certain sedimentary conditions will reflect changes in past biological productivity and/or deep ocean ventilation. Applications for the isotopic composition of U (234U:238U activity ratio, δ234U or the per mil deviation from secular equilibrium) as a paleoceanographic proxy, however, remain largely unexplored in pelagic sediments. We present δ234U of bulk U for a 504-kyr record of South Atlantic core ODP 1094, a record that has exhibited sensitivity to bottom water formation with minimal 238U remobilization due to changes in redox condition. Sedimentary δ234U at time of burial was modeled given authigenic and detrital sources, where it was found that δ234U has a reoccurring pattern of anomalous behavior around deglacial transitions. In many cases, the anomalous δ234U compositions in sediment exceeded seawater, which is not consistent with its expected sources. A mass balance of 234U across these anomalous sections showed that vertical rearrangement of 234U can explain anomalous sedimentary δ234U during Marine Isotope Stage (MIS) 7/8 and 11/12 transitions, as well as partially during MIS 5/6 and MIS 9/10. The distribution of these anomalies is likely consistent with the idea that deglacial spikes in aU, resulting from periods of high organic carbon delivery and thereby strong reduction in the sediments, act as a persistent sink of 234U alpha-recoiled from vertically adjacent sediment layers after burial. While this mechanism explains the negative δ234U anomalies, there is a large subset of positive δ234U anomalies that cannot be explained by vertical diffusion of 234U in pore waters alone. This excess 234U points to an extremely high δ234U source to seawater during deglaciation or additional diagenetic effects such as diffusion from lateral heterogeneity in the redox state of sediment at depth. We find that δ234U of pelagic sediment may be useful to identify fine-scale pore water migration of alpha-recoil products.

Five sediment cores were collected along a cruise tract from Hawaii to Alaska in August 2017 (C-Disk-IV cruise) with the objective of characterizing the behavior of Ac-227, Ra-228, and Ra-226 and their fluxes into the overlying water column, information that is essential to the interpretation of the distribution of these tracers in the ocean, for example, as measured on GEOTRACES cruises. Solid phase profiles of these isotopes were measured, and reaction-transport models were applied that incorporated molecular diffusion, bioturbation, sedimentation, distribution coefficients (kd), and the fraction of each isotope released to pore water by parent decay (called F). Fits to these profiles used kd values determined in lab experiments for C-Disk-IV sediments. Ra kd values (1000-3000 mL g(-1)) agreed with previous estimates for deep-sea sediments, and Ac kd values (3500-22,000 mL g(-1)) correlated with those for Ra but were about 7 times greater. Two independent approaches were used to quantify the benthic fluxes of Ac-227 and 228Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agreed within uncertainty, and the average Ac-227 and Ra-228 sediment fluxes for the Northeast Pacific are 90 & PLUSMN; 20 and 600 &200 dpm m-2-yr(-1), respectively. The 226Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific, averaging 1300 & PLUSMN; 200 dpm m-2-yr(-1). This is over 2x higher than the water column inventory of Ra-226 in this region (600 dpm m-2-yr(-1)), and indicates the importance of lateral 226Ra export from the N. Pacific. The largest 227Ac and Ra isotope fluxes in the study area are near the center of the Northeast Pacific (37 degrees N). Smaller Ac-227, Ra-228 and Ra-226 fluxes occur north of 40 degrees N, primarily due to dilution of their Pa and Th ancestors by higher sediment accumulation rates.

A technique is developed to quantify the ultra-trace 231 Pa (35–3904 ag) concentration in seawater using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The method is a modification of the process developed by Shen et al. (Anal Chem 75(5):1075–1079, 2003. https://doi.org/10.1021/ac026247r ) and extends it to the application of very low levels of actinides, and the 35 ag 231 Pa can be measured with a precision of 15%. The total process blank for the water column was 0.02 ag/g, while the values of the large and small particles were ~ 30 ag/g. The ionization efficiency (ions generated/atom loaded) varies from 0.7 to 2.4%. The measurement time is 2–5 min. The amount of 231 Pa needed to produce 231 Pa data with an uncertainty of ± 0.8–15% is 35–3904 ag (~ 0.9 × 10 5 to 10 × 10 6 atoms). Replicate measurements of known standards and seawater samples demonstrate that the analytical precision approximates that expected from counting statistics, and that based on detection limits of 52 ag, 55 ag, and 28 ag, protactinium can be detected in a minimum seawater sample size of ~ 2.6 L for small suspended particulate matter (> 0.8 μm and < 51 μm), ~ 3.0 L for large suspended particulate matter (> 51 μm), and ~ 56 mL for filtered (< 0.45 μm) seawater. The concentration of 231 Pa (several attograms per liter) can be determined with an uncertainty of ± 2–8% (2 σ ) for suspended particulate matter filtered from ~ 60 L of seawater. For the dissolved fraction, ~ 1 L of seawater yields 231 Pa measurements with a precision of 0.8–10%. The sample size requirements are several orders of magnitude less than traditional decay-counting techniques, and the precision is better than that previously reported for ICP-MS techniques. Our technique can also be applied to other environmental samples, including river, lake, and cave water samples.

Ice core measurements show diverse atmospheric CO 2 variations—increasing, decreasing or remaining stable—during millennial-scale North Atlantic cold periods called stadials. The reasons for these contrasting trends remain elusive. Ventilation of carbon-rich deep oceans can profoundly affect atmospheric CO 2 , but its millennial-scale history is poorly constrained. Here we present a well-dated high-resolution deep Atlantic acidity record over the past 150,000 years, which reveals five hitherto undetected modes of stadial ocean ventilation with different consequences for deep-sea carbon storage and associated atmospheric CO 2 changes. Our data provide observational evidence to show that strong and often volumetrically extensive Southern Ocean ventilation released substantial amounts of deep-sea carbon during stadials when atmospheric CO 2 rose prominently. By contrast, other stadials were characterized by weak ventilation via both Southern Ocean and North Atlantic, which promoted respired carbon accumulation and thus curtailed or reversed deep-sea carbon losses, resulting in diminished rises or even declines in atmospheric CO 2 . Our findings demonstrate that millennial-scale changes in deep-sea carbon storage and atmospheric CO 2 are modulated by multiple ocean ventilation modes through the interplay of the two polar regions, rather than by the Southern Ocean alone, which is critical for comprehensive understanding of past and future carbon cycle adjustments to climate change.

Journal Article Politics and the People: Scotland, 1945–1979. By Malcolm R. Petrie. Who Runs Edinburgh? By David McCrone Get access Politics and the People: Scotland, 1945–1979. By Malcolm R. Petrie. Edinburgh University Press, Edinburgh, 2022. x + 213 pp. ISBN 978-1-4744-5698-2, £85.Who Runs Edinburgh? By David McCrone. Edinburgh University Press, Edinburgh, 2022. xii + 299 pp. ISBN 978-1-4744-9831-9 (paperback), £14.99. Robert Anderson Robert Anderson University of Edinburgh R.D.Anderson@ed.ac.uk Search for other works by this author on: Oxford Academic Google Scholar Twentieth Century British History, hwad041, https://doi.org/10.1093/tcbh/hwad041 Published: 31 May 2023