Geochemical evidence of a floating Arctic ice sheet and underlying freshwater in the Arctic Mediterranean in glacial periods

Numerous studies have addressed the possible existence of large floating ice sheets in the glacial Arctic Ocean from theoretical, modelling, or seafloor morphology perspectives. Here, we add evidence from the sediment record that support the existence of such freshwater ice caps in certain intervals, and we discuss their implications for possible non-linear and rapid behaviour of such a system in the high latitudes. We present sedimentary activities of 230Th together with 234U/238U ratios, the concentrations of manganese, sulphur and calcium in the context of lithological information and records of microfossils and their isotope composition. New analyses (PS51/038, PS72/396) and a re-analysis of existing marine sediment records (PS1533, PS1235, PS2185, PS2200, amongst others) in view of the naturally occurring radionuclide 230Thex and, where available, 10Be from the Arctic Ocean and the Nordic Seas reveal the widespread occurrence of intervals with a specific geochemical signature. The pattern of these parameters in a pan-Arctic view can best be explained when assuming the repeated presence of freshwater in frozen and liquid form across large parts of the Arctic Ocean and the Nordic Seas. Based on the sedimentary evidence and known environmental constraints at the time, we develop a glacial scenario that explains how these ice sheets, together with eustatic sea-level changes, may have affected the past oceanography of the Arctic Ocean in a fundamental way that must have led to a drastic and non-linear response to external forcing. This concept offers a possibility to explain and to some extent reconcile contrasting age models for the Late Pleistocene in the Arctic Ocean. Our view, if adopted, offers a coherent dating approach across the Arctic Ocean and the Nordic Seas, linked to events outside the Arctic

Carbon dynamics of the Weddell Gyre, Southern Ocean

The accumulation of carbon within the Weddell Gyre and its exchanges across the gyre boundaries are investigated with three recent full-depth oceanographic sections enclosing this climatically important region. The combination of carbonmeasurements with ocean circulation transport estimates from a box inverse analysis reveals that deepwater transports associated with Warm Deep Water (WDW) and Weddell Sea Deep Water dominate the gyre’s carbon budget, while a dual-cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean. Historical sea surface pCO2 observations, interpolated using a neural network technique, confirm the net summertime sink of 0.044 to 0.058 ± 0.010 Pg C / yr derived from the inversion. However, a wintertime outgassing signal similar in size results in a statistically insignificant annual air-to-sea CO2 flux of 0.002± 0.007 Pg C / yr (mean 1998–2011) to 0.012 ± 0.024 Pg C/ yr (mean 2008–2010) to be diagnosed for the Weddell Gyre. A surface layer carbon balance, independently derived fromin situ biogeochemical measurements, reveals that freshwater inputs and biological drawdown decrease surface ocean inorganic carbon levels more than they are increased by WDW entrainment, resulting in an estimated annual carbon sink of 0.033 ± 0.021 Pg C / yr. Although relatively less efficient for carbon uptake than the global oceans, the summertime Weddell Gyre suppresses the winter outgassing signal, while its biological pump and deepwater formation act as key conduits for transporting natural and anthropogenic carbon to the deep ocean where they can reside for long time scales

Silica burial enhanced by iron limitation in oceanic upwelling margins

In large swaths of the ocean, primary production by diatoms may be limited by the availability of silica, which in turn limits the biological uptake of carbon dioxide. The burial of biogenic silica in the form of opal is the main sink of marine silicon. Opal burial occurs in equal parts in iron-limited open-ocean provinces and upwelling margins, especially the eastern Pacific upwelling zone. However, it is unclear why opal burial is so efficient in this margin. Here we measure fluxes of biogenic material, concentrations of diatom-bound iron and silicon isotope ratios using sediment traps and a sediment core from the Gulf of California upwelling margin. In the sediment trap material, we find that periods of intense upwelling are associated with transient iron limitation that results in a high export of silica relative to organic carbon. A similar correlation between enhanced silica burial and iron limitation is evident in the sediment core, which spans the past 26,000 years. A global compilation also indicates that hotspots of silicon burial in the ocean are all characterized by high silica to organic carbon export ratios, a diagnostic trait for diatoms growing under iron stress. We therefore propose that prevailing conditions of silica limitation in the ocean are largely caused by iron deficiency imposing an indirect constraint on oceanic carbon uptake

Comment on "Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?" by M. Lyle et al.

Accurately estimating the vertical flux of material reaching the seafloor from the overlying surface waters is essential for the paleoceanographic reconstruction of a wide variety of oceanic processes. Two approaches are currently being used. One consists of estimating mass accumulation rates (MAR) between dated horizons as the product of linear sedimentation rates, sediment dry bulk densities, and concentrations. One pitfall with this approach is that sediments can be redistributed on the seafloor by bottom currents, and their accumulation may not necessarily reflect the true vertical rain rate originating from the overlying water column. To address this problem, the method of ²³⁰Th normalization was developed [Bacon, 1984]. This method is based on the assumption that the rapid scavenging of ²³⁰Th produced in the water column by decay of dissolved uranium results in its flux to the seafloor always being close to its known rate of production. To the extent that this assumption is correct, scavenged ²³⁰Th can be used as a reference to estimate the settling flux of other sedimentary constituents and to correct for sediment redistribution on the seafloor [Henderson and Anderson, 2003; Francois et al., 2004]

Nutrient cycling in the Atlantic basin: the evolution of nitrate isotope signatures in water masses

A basin-wide transect of nitrate isotopes (δ15NNO3, δ18ONO3), across the UK GEOTRACES 40°S transect in the South Atlantic is presented. This data set is used to investigate Atlantic nutrient cycling and the communication pathways of nitrogen cycling processes in the global ocean. Intermediate waters formed in the subantarctic are enriched in δ15NNO3 and δ18ONO3 from partial utilization of nitrate by phytoplankton and distant denitrification processes, transporting heavy isotope signatures to the subtropical Atlantic. Water mass modification through the Atlantic is investigated by comparing data from 40°S (South Atlantic) and 30°N (North Atlantic). This reveals that nitrate in the upper intermediate waters is regenerated as it transits through the subtropical Atlantic, as evidenced by decreases in δ18ONO3. We document diazotrophy-producing high N:P particle ratios (18–21:1) for remineralization, which is further confirmed by a decrease in δ15NNO3 through the subtropical Atlantic. Thesemodifications influence the isotopic signatures of the North Atlantic Deep Water (NADW) which is subsequently exported from the Atlantic to the Southern Ocean. This study reveals the dominance of recycling processes and diazotrophy on nitrate cycling in the Atlantic. These processes provide a source of low δ15NNO3 to the Southern Ocean via the NADW, to counteract enrichment in δ15NNO3 from water column denitrification in the Indo/Pacific basins. We hence identify the Southern Ocean as a key hub through which denitrification and N2 fixation communicate in the ocean through deepwater masses. Therefore, the balancing of the oceanic N budget and isotopic signatures require time scales of oceanic mixing

Perspectives and Integration in SOLAS Science

Why a chapter on Perspectives and Integration in SOLAS Science in this book? SOLAS science by its nature deals with interactions that occur: across a wide spectrum of time and space scales, involve gases and particles, between the ocean and the atmosphere, across many disciplines including chemistry, biology, optics, physics, mathematics, computing, socio-economics and consequently interactions between many different scientists and across scientific generations. This chapter provides a guide through the remarkable diversity of cross-cutting approaches and tools in the gigantic puzzle of the SOLAS realm. Here we overview the existing prime components of atmospheric and oceanic observing systems, with the acquisition of ocean–atmosphere observables either from in situ or from satellites, the rich hierarchy of models to test our knowledge of Earth System functioning, and the tremendous efforts accomplished over the last decade within the COST Action 735 and SOLAS Integration project frameworks to understand, as best we can, the current physical and biogeochemical state of the atmosphere and ocean commons. A few SOLAS integrative studies illustrate the full meaning of interactions, paving the way for even tighter connections between thematic fields. Ultimately, SOLAS research will also develop with an enhanced consideration of societal demand while preserving fundamental research coherency. The exchange of energy, gases and particles across the air-sea interface is controlled by a variety of biological, chemical and physical processes that operate across broad spatial and temporal scales. These processes influence the composition, biogeochemical and chemical properties of both the oceanic and atmospheric boundary layers and ultimately shape the Earth system response to climate and environmental change, as detailed in the previous four chapters. In this cross-cutting chapter we present some of the SOLAS achievements over the last decade in terms of integration, upscaling observational information from process-oriented studies and expeditionary research with key tools such as remote sensing and modelling. Here we do not pretend to encompass the entire legacy of SOLAS efforts but rather offer a selective view of some of the major integrative SOLAS studies that combined available pieces of the immense jigsaw puzzle. These include, for instance, COST efforts to build up global climatologies of SOLAS relevant parameters such as dimethyl sulphide, interconnection between volcanic ash and ecosystem response in the eastern subarctic North Pacific, optimal strategy to derive basin-scale CO2 uptake with good precision, or significant reduction of the uncertainties in sea-salt aerosol source functions. Predicting the future trajectory of Earth’s climate and habitability is the main task ahead. Some possible routes for the SOLAS scientific community to reach this overarching goal conclude the chapter

High primary productivity in an ice melting hot spot at the eastern boundary of the Weddell Gyre

The Southern Ocean (SO) plays a key role in modulating atmospheric CO 2 via physical and biological processes. However, over much of the SO, biological activity is iron-limited. New in situ data from the Antarctic zone south of Africa in a region centered at ~20°E-25°E reveal a previously overlooked region of high primary production, comparable in size to the northwest African upwelling region. Here, sea ice together with enclosed icebergs is channeled by prevailing winds to the eastern boundary of the Weddell Gyre, where a sharp transition to warmer waters causes melting. This cumulative melting provides a steady source of iron, fuelling an intense phytoplankton bloom that is not fully captured by monthly satellite production estimates. These findings imply that future changes in sea-ice cover and dynamics could have a significant effect on carbon sequestration in the SO