Carbon isotope evidence for a northern source of deep water in the glacial western North Atlantic.
ABSTRACT: The prevailing view of western Atlantic hydrography during the Last Glacial Maximum (LGM) calls for transport and intermixing of deep southern and intermediate northern end members. However, ?13C and ?14C results on foraminifera from a sediment core at 5.0 km in the northern subtropics show that there may have also been a northern source of relatively young, very dense, nutrient-depleted water during the LGM (18 ky to 21 ky ago). These results, when integrated with data from other western North Atlantic locations, indicate that the ocean was poorly ventilated at 4.2 km, with better ventilation above and below that depth. If this is a signal of water mass source and not nutrient storage, it would indicate that a previously unrecognized deep water end member originated along the western margin of the Labrador Sea, analogous to dense water formation today around Antarctica and in the Okhotsk Sea.
Project description:The coast of northern Primorye region, north of Peter the Great Bay has been sparsely studied in regards to its molluscan fauna, with just a few works reviewing the distribution of local mollusks. This work presents a survey of the shallow water heterobranch sea slugs currently occurring around Kievka Bay to Oprichnik Bay, Russia. Thirty-nine species of sea slugs were found in this study and the new species <i>Cadlina olgae</i> sp. nov., described herein. Most (24) of the species occurring in the area have widespread ranges in the northern Pacific Ocean. The eight species are endemic for the Sea of Japan and adjacent part of the Sea of Okhotsk. Seven other occur also in northern Atlantic and Arctic waters. Thirteen found species are not known from Peter the Great Bay but known from adjacent northern Pacific waters. The finding of a previously undescribed species emphasizes the need of further surveys, particularly in subtidal and deeper waters, in order to improve the knowledge on this neglected fauna in Primorye.
Project description:The Nd isotope composition of seawater has been used to reconstruct past changes in the contribution of different water masses to the deep ocean. In the absence of contrary information, the Nd isotope compositions of endmember water masses are usually assumed constant during the Quaternary. Here we show that the Nd isotope composition of North Atlantic Deep Water (NADW), a major component of the global overturning ocean circulation, was significantly more radiogenic than modern during the Last Glacial Maximum (LGM), and shifted towards modern values during the deglaciation. We propose that weathering contributions of unradiogenic Nd modulated by the North American Ice Sheet dominated the evolution of the NADW Nd isotope endmember. If water mass mixing dominated the distribution of deep glacial Atlantic Nd isotopes, our results would imply a larger fraction of NADW in the deep Atlantic during the LGM and deglaciation than reconstructed with a constant northern endmember.
Project description:Iron is one of the key elements controlling phytoplankton growth in large areas of the global ocean. Aeolian dust has traditionally been considered the major external source of iron in the North Pacific. Recent studies have indicated that sedimentary iron from the shelf region of the Sea of Okhotsk has a strong impact on the iron distribution in the North Pacific, while the mechanism supporting its long-distance transport remains poorly understood. Here, we report that refractory shelf humic substances, which complex and carry dissolved iron, are transported conservatively at least 4000?km from the shallow sediments of the Sea of Okhotsk to the subtropical North Pacific with the circulation of intermediate water. This result indicates that shelf humic substances are probably one of the key factors shaping the distribution of dissolved iron in the ocean interior.
Project description:Warm subtropical-origin Atlantic water flows northward across the Greenland-Scotland Ridge into the Nordic Seas, where it relinquishes heat to the atmosphere and gradually transforms into dense Atlantic-origin water. Returning southward along east Greenland, this water mass is situated beneath a layer of cold, fresh surface water and sea ice. Here we show, using measurements from autonomous ocean gliders, that the Atlantic-origin water was re-ventilated while transiting the western Iceland Sea during winter. This re-ventilation is a recent phenomenon made possible by the retreat of the ice edge toward Greenland. The fresh surface layer that characterises this region in summer is diverted onto the Greenland shelf by enhanced onshore Ekman transport induced by stronger northerly winds in fall and winter. Severe heat loss from the ocean offshore of the ice edge subsequently triggers convection, which further transforms the Atlantic-origin water. This re-ventilation is a counterintuitive occurrence in a warming climate, and highlights the difficulties inherent in predicting the behaviour of the complex coupled climate system.
Project description:The end of the Last Glacial Maximum (LGM) dramatically reshaped temperate ecosystems, with many species moving poleward as temperatures rose and ice receded. Whereas reinvading terrestrial taxa tracked melting glaciers, marine biota recolonized ocean habitats freed by retreating sea ice. The extent of sea ice in the Southern Hemisphere during the LGM has, however, yet to be fully resolved, with most palaeogeographic studies suggesting only minimal or patchy ice cover in subantarctic waters. Here, through population genetic analyses of the widespread Southern Bull Kelp (Durvillaea antarctica), we present evidence for persistent ice scour affecting subantarctic islands during the LGM. Using mitochondrial and chloroplast genetic markers (COI; rbcL) to genetically characterize some 300 kelp samples from 45 Southern Ocean localities, we reveal a remarkable pattern of recent recolonization in the subantarctic. Specifically, in contrast to the marked phylogeographic structure observed across coastal New Zealand and Chile (10- to 100-km scales), subantarctic samples show striking genetic homogeneity over vast distances (10,000-km scales), with a single widespread haplotype observed for each marker. From these results, we suggest that sea ice expanded further and ice scour during the LGM impacted shallow-water subantarctic marine ecosystems more extensively than previously suggested.
Project description:The phylogenetic diversity of ammonia-oxidizing archaea (AOA) was surveyed in the surface sediments from the northern part of the South China Sea (SCS). The distribution pattern of AOA in the western Pacific was discussed through comparing the SCS with other areas in the western Pacific including Changjiang Estuary and the adjacent East China Sea where high input of anthropogenic nitrogen was evident, the tropical West Pacific Continental Margins close to the Philippines, the deep-sea methane seep sediments in the Okhotsk Sea, the cold deep sea of Northeastern Japan Sea, and the hydrothermal field in the Southern Okinawa Trough. These various environments provide a wide spectrum of physical and chemical conditions for a better understanding of the distribution pattern and diversities of AOA in the western Pacific. Under these different conditions, the distinct community composition between shallow and deep-sea sediments was clearly delineated based on the UniFrac PCoA and Jackknife Environmental Cluster analyses. Phylogenetic analyses showed that a few ammonia-oxidizing archaeal subclades in the marine water column/sediment clade and endemic lineages were indicative phylotypes for some environments. Higher phylogenetic diversity was observed in the Philippines while lower diversity in the hydrothermal vent habitat. Water depth and possibly with other environmental factors could be the main driving forces to shape the phylogenetic diversity of AOA observed, not only in the SCS but also in the whole western Pacific. The multivariate regression tree analysis also supported this observation consistently. Moreover, the functions of current and other climate factors were also discussed in comparison of phylogenetic diversity. The information collectively provides important insights into the ecophysiological requirements of uncultured ammonia-oxidizing archaeal lineages in the western Pacific Ocean.
Project description:Producing independent and accurate chronologies for marine sediments is a prerequisite to understand the sequence of millennial-scale events and reveal potential temporal offsets between marine and continental records, or between different marine records, possibly from different regions. The last 40 ky is a generally well-constrained period since radiocarbon (14C) can be used as an absolute dating tool. However, in the northern North Atlantic, calendar ages cannot be directly derived from 14C ages, due to temporal and spatial variations of surface reservoir ages. Alternatively, chronologies can be derived by aligning Greenland ice-core time series with marine surface records. Yet this approach suffers from the lack of clearly defined climatic events between 14.7 and 23.3 cal ky BP (hereafter ka), a crucial period encompassing Heinrich Stadial 1 and the onset of the last deglaciation. In this study, (i) we assess the benefits of 230Th normalization to refine the sedimentation history between surface temperature alignment tie points and (ii) revisit the chronologies of three North Atlantic marine records. Our study supports the contention that the marked increase in the Greenland Ca2+ record at 17.48 ka ± 0.21 ky (1?) occurred within dating uncertainty of sea surface temperature cooling in the North Atlantic at the onset of Heinrich Stadial 1. This sharp feature might be useful for future chronostratigraphic alignments to remedy the lack of chronological constraint between 14.7 and 23.3 ka for North Atlantic marine records that are subject to large changes in 14C surface reservoir age.
Project description:The Red Sea hosts a deep marine environment unique among the world's oceans. It is occupied, almost homogeneously from the subsurface (~137 to 300 m) to depths over 2000 m, by a warm (~21.5°C) and highly saline (~40.5) water mass, referred to as the Red Sea Deep Water (RSDW). Previous studies suggested that the RSDW is mainly ventilated, continuously or intermittently, by dense outflows from the northern Gulfs of Suez and Aqaba with a resulting sluggish renewal time on the order of 36 to 90 years. We use six repeated hydrographic observations spanning the period 1982-2011 and simulations of an ocean general circulation model with realistic atmospheric forcing to show that large portions of the RSDW were episodically replaced during 1982-2001 by new dense waters mainly formed by open-ocean deep convections in the northern Red Sea during anomalously cold winters, pointing to a much shorter renewal time for the RSDW on the order of a decade. We further show that the winter cooling anomaly in the Red Sea region was a part of a large-scale climate variability pattern associated with either large volcanic eruptions or the North Atlantic Oscillation (NAO). Consequently, significant deep water formation events occurred in the Red Sea in the winters following the 1982 El Chichón eruption in Mexico and the 1991 Mount Pinatubo eruption in the Philippines and during the strong positive phase of the NAO in the winter of 1989.
Project description:Using a novel combination of palaeohabitat modelling and genetic mixture analyses, we identify and assess a sea-level-driven recolonization process following the Last Glacial Maximum (LGM). Our palaeohabitat modelling reveals dramatic changes in estuarine habitat distribution along the coast of California (USA) and Baja California (Mexico). At the LGM (approx. 20 kya), when sea level was approximately 130 m lower, the palaeo-shoreline was too steep for tidal estuarine habitat formation, eliminating this habitat type from regions where it is currently most abundant, and limiting such estuaries to a northern and a southern refugium separated by 1000 km. We assess the recolonization of estuaries formed during post-LGM sea-level rise through examination of refugium-associated alleles and approximate Bayesian computation in three species of estuarine fishes. Results reveal sourcing of modern populations from both refugia, which admix in the newly formed habitat between the refuges. We infer a dramatic peak in habitat area between 15 and 10 kya with subsequent decline. Overall, this approach revealed a previously undocumented dynamic and integrated relationship between sea-level change, coastal processes and population genetics. These results extend glacial refugial dynamics to unglaciated subtropical coasts and have significant implications for biotic response to predicted sea-level rise.
Project description:BACKGROUND: Understanding the dynamics of the human range expansion across northeastern Eurasia during the late Pleistocene is central to establishing empirical temporal constraints on the colonization of the Americas. Opinions vary widely on how and when the Americas were colonized, with advocates supporting either a pre- or post- last glacial maximum (LGM) colonization, via either a land bridge across Beringia, a sea-faring Pacific Rim coastal route, a trans-Arctic route, or a trans-Atlantic oceanic route. Here we analyze a large sample of radiocarbon dates from the northeast Eurasian Upper Paleolithic to identify the origin of this expansion, and estimate the velocity of colonization wave as it moved across northern Eurasia and into the Americas. METHODOLOGY/PRINCIPAL FINDINGS: We use diffusion models to quantify these dynamics. Our results show the expansion originated in the Altai region of southern Siberia approximately 46kBP , and from there expanded across northern Eurasia at an average velocity of 0.16 km per year. However, the movement of the colonizing wave was not continuous but underwent three distinct phases: 1) an initial expansion from 47-32k calBP; 2) a hiatus from approximately 32-16k calBP, and 3) a second expansion after the LGM approximately 16k calBP. These results provide archaeological support for the recently proposed three-stage model of the colonization of the Americas. Our results falsify the hypothesis of a pre-LGM terrestrial colonization of the Americas and we discuss the importance of these empirical results in the light of alternative models. CONCLUSIONS/SIGNIFICANCE: Our results demonstrate that the radiocarbon record of Upper Paleolithic northeastern Eurasia supports a post-LGM terrestrial colonization of the Americas falsifying the proposed pre-LGM terrestrial colonization of the Americas. We show that this expansion was not a simple process, but proceeded in three phases, consistent with genetic data, largely in response to the variable climatic conditions of late Pleistocene northeast Eurasia. Further, the constraints imposed by the spatiotemporal gradient in the empirical radiocarbon record across this entire region suggests that North America cannot have been colonized much before the existing Clovis radiocarbon record suggests.