Observations of dissolved iron concentrations in the World Ocean: implications and constraints for ocean biogeochemical models

International audienceAnalysis of a global compilation of dissolved iron observations provides insights into the controlling processes for iron distributions and some constraints for ocean biogeochemical models. The distribution of dissolved iron is consistent with the conceptual model developed for the scavenging of Th isotopes, whereby particle scavenging is a two-step process of scavenging mainly by colloidal and small particulates followed by aggregation and removal on larger sinking particles. Much of the dissolved iron (~0.02 ?m) and, thus, likely subject to aggregation and scavenging removal. Only the iron bound to soluble ligands ( Inputs from dust deposition and continental sediments are key drivers of dissolved iron distributions. The observations provide several strong constraints for ocean biogeochemical models: 1) similar deep ocean concentrations in the North Atlantic and North Pacific (~0.6?0.8 nM), and much lower deep ocean dissolved iron concentrations in the Southern Ocean (~0.3?0.4 nM); 2) strong depletion of iron in the upper ocean away from the high dust deposition regions, with significant scavenging removal of dissolved iron below the euphotic zone; and 3) a bimodal distribution in surface waters with peaks less than 0.2 nM and between 0.6?0.8 nM. We compare the dissolved iron observations with output from the Biogeochemical Elemental Cycling (BEC) ocean model. The model output was in general agreement with the field data (r=0.76, for depths 103?502 m), but at lower iron concentrations (<0.3 nM) the model is consistently biased high relative to the observations

Age and date for early arrival of the Acheulian in Europe (Barranc de la Boella, la Canonja, Spain)

The first arrivals of hominin populations into Eurasia during the Early Pleistocene are currently considered to have occurred as short and poorly dated biological dispersions. Questions as to the tempo and mode of these early prehistoric settlements have given rise to debates concerning the taxonomic significance of the lithic assemblages, as trace fossils, and the geographical distribution of the technological traditions found in the Lower Palaeolithic record. Here, we report on the Barranc de la Boella site which has yielded a lithic assemblage dating to ,1 million years ago that includes large cutting tools (LCT). We argue that distinct technological traditions coexisted in the Iberian archaeological repertoires of the late Early Pleistocene age in a similar way to the earliest sub-Saharan African artefact assemblages. These differences between stone tool assemblages may be attributed to the different chronologies of hominin dispersal events. The archaeological record of Barranc de la Boella completes the geographical distribution of LCT assemblages across southern Eurasia during the EMPT (Early-Middle Pleistocene Transition, circa 942 to 641 kyr). Up to now, chronology of the earliest European LCT assemblages is based on the abundant Palaeolithic record found in terrace river sequences which have been dated to the end of the EMPT and later. However, the findings at Barranc de la Boella suggest that early LCT lithic assemblages appeared in the SW of Europe during earlier hominin dispersal episodes before the definitive colonization of temperate Eurasia took place.The research at Barranc de la Boella has been carried out with the financial support of the Spanish Ministerio de Economı´a y Competitividad (CGL2012- 36682; CGL2012-38358, CGL2012-38434-C03-03 and CGL2010-15326; MICINN project HAR2009-7223/HIST), Generalitat de Catalunya, AGAUR agence (projects 2014SGR-901; 2014SGR-899; 2009SGR-324, 2009PBR-0033 and 2009SGR-188) and Junta de Castilla y Leo´n BU1004A09. Financial support for Barranc de la Boella field work and archaeological excavations is provided by the Ajuntament de la Canonja and Departament de Cultura (Servei d’Arqueologia i Paleontologia) de la Generalitat de Catalunya. A. Carrancho’s research was funded by the International Excellence Programme, Reinforcement subprogramme of the Spanish Ministry of Education. I. Lozano-Ferna´ndez acknowledges the pre-doctoral grant from the Fundacio´n Atapuerca. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Sedimentary and mineral dust sources of dissolved iron to the world ocean

Analysis of a global compilation of dissolved-iron observations provides insights into the processes controlling iron distributions and some constraints for ocean biogeochemical models. The distribution of dissolved iron appears consistent with the conceptual model developed for Th isotopes, whereby particle scavenging is a two-step process of scavenging mainly by colloidal and small particulates, followed by aggregation and removal on larger sinking particles. Much of the dissolved iron (&lt;0.4 μm) is present as small colloids (&gt;~0.02 μm) and, thus, is subject to aggregation and scavenging removal. This implies distinct scavenging regimes for dissolved iron consistent with the observations: 1) a high scavenging regime – where dissolved-iron concentrations exceed the concentrations of strongly binding organic ligands; and 2) a moderate scavenging regime – where dissolved iron is bound to both colloidal and soluble ligands. Within the moderate scavenging regime, biological uptake and particle scavenging decrease surface iron concentrations to low levels (&lt;0.2 nM) over a wide range of low to moderate iron input levels. Removal rates are also highly nonlinear in areas with higher iron inputs. Thus, observed surface-iron concentrations exhibit a bi-modal distribution and are a poor proxy for iron input rates. Our results suggest that there is substantial removal of dissolved iron from subsurface waters (where iron concentrations are often well below 0.6 nM), most likely due to aggregation and removal on sinking particles of Fe bound to organic colloids. We use the observational database to improve simulation of the iron cycle within a global-scale, Biogeochemical Elemental Cycling (BEC) ocean model. Modifications to the model include: 1) an improved particle scavenging parameterization, based on the sinking mass flux of particulate organic material, biogenic silica, calcium carbonate, and mineral dust particles; 2) desorption of dissolved iron from sinking particles; and 3) an improved sedimentary source for dissolved iron. Most scavenged iron (90%) is put on sinking particles to remineralize deeper in the water column. The model-observation differences are reduced with these modifications. The improved BEC model is used to examine the relative contributions of mineral dust and marine sediments in driving dissolved-iron distributions and marine biogeochemistry. Mineral dust and sedimentary sources of iron contribute roughly equally, on average, to dissolved iron concentrations. The sedimentary source from the continental margins has a strong impact on open-ocean iron concentrations, particularly in the North Pacific. Plumes of elevated dissolved-iron concentrations develop at depth in the Southern Ocean, extending from source regions in the SW Atlantic and around New Zealand. The lower particle flux and weaker scavenging in the Southern Ocean allows the continental iron source to be advected far from sources. Both the margin sediment and mineral dust Fe sources substantially influence global-scale primary production, export production, and nitrogen fixation, with a stronger role for the dust source. Ocean biogeochemical models that do not include the sedimentary source for dissolved iron, will overestimate the impact of dust deposition variations on the marine carbon cycle. Available iron observations place some strong constraints on ocean biogeochemical models. Model results should be evaluated against both surface and subsurface Fe observations in the waters that supply dissolved iron to the euphotic zone

Cosmogenic 10Be production rate calibrated against 3He in the high Tropical Andes (3800–4900 m, 20–22° S)

International audienceMany geomorphologic applications, notably glacier chronologies, require improvements in both the precision and the accuracy of the cosmogenic dating tool. Of particular importance is the need to better constrain the spatial variability of the cosmogenic nuclides production rates at high elevation and low latitudes. One strategy that can be adopted for this is to couple absolute calibrations, from independently dated surfaces, with cross-calibration studies, performed by measuring several cosmogenic nuclides in the same rock.In the present study, we report the highest-elevation (>4800 m) cross-calibration published to date, comprising measurements of cosmogenic 3He and 10Be in cogenetic pyroxene and quartz. The samples were collected from six dacitic moraine boulders, exposed from 32 to 65 ka at 4820 m on the flanks of the Uturuncu volcano (22° S, 67° W), Southern Lipez (Bolivia). The samples yield a remarkably tight cluster of 3He–10Be production ratios, with a weighted mean of 33.3 ± 0.9 (1σ). This production ratio is undistinguishable, within uncertainties, from the 3He–10Be production ratio of 32.3 ± 0.9 determined in the same mineral pair at low elevation (1333 m) by Amidon et al. (2009). These results agree at the 1σ level and suggest that any hypothetical increase of the 3He–10Be production ratio in pyroxene and quartz is likely to be lower than 5% over this elevation range (1000–5000 m). Moreover, the production ratio is almost insensitive to the Li content of the pyroxene (20 to 50 ppm Li), suggesting that the cosmogenic thermal neutron production of 3He is very low in this setting.The high-elevation 3He–10Be production ratio is used in combination with a local determination of the 3He production rate in the high Central Altiplano (3800 m) (Blard et al., 2013) to establish a local 10Be production rate of 30.0 ± 1.4at g-1 yr-1 at 3800 m and 20° S. After scaling to sea-level high latitude with the time-dependent Lal/Stone model, this yields a 10Be production rate in quartz of 3.63 ± 0.17at g-1 yr-1 Importantly, this rate can be used for high-precision geomorphologic dating, for example for determining glacial chronologies ( 1σ < 4%) through 10Be dating of moraines in the high Tropical Andes. Any inaccuracy attached to the scaling model is canceled out when the calibration site is located close to the dated object.The same experiment was also undertaken in pyroxene and plagioclase from two andesitic boulders from the Tunupa volcano moraines, exposed at 3800 m and 4200 m. A 3He–10Be production ratio of 35.9 ± 1.3 (1σ) is obtained for this mineral pair, indicating that the 10Be production rate in plagioclase is about 8% lower than in quartz

Late Quaternary ice sheet extents in northeastern Germany inferred from surface exposure dating

Quaternary Glaciation History of Northern EuropeInternational audienceno abstrac

Slope instability in relation to glacial debuttressing in alpine areas (Upper Durance catchment, southeastern France): Evidence from field data and 10Be cosmic ray exposure ages

International audienceThe Upper Durance catchment is an area prone to rock-slope failures. Such failures reflect the combination of high relief, litho-structural controls and paraglacial stress release. The aim of this study is to determine the role of deglacial unloading and resulting paraglacial stress release in conditioning or triggering slope failure. Former dimensions of the Durance glacier are reconstructed, then combined with Digital Elevation Model data in a raster Geographic Information System to quantify the spatial pattern of stresses associated with glacial loading at the Last Glacial Maximum. Preliminary calculations suggest that major rock falls and rock avalanches are associated with areas subject to the highest decompression stresses. Focus on two case studies allows the consequences of paraglacial stress release on slope instability to be evaluated. Description of slope failure runout deposits allows reconstruction of the nature of slope failure. Surface exposure dates based on concentration of cosmogenic 10Be allows the timing of both deglaciation and that of post-glacial rock-slope failures to be established. It is shown that rock-slope failures are concentrated on lower valley-side slopes within the area occupied by ice at the Last Glacial Maximum, and that their locations coincide with zones of inferred high glacial loading stress, consistent with interpretation of both bedrock disruption and large-scale rock-slope failures as paraglacial phenomena induced by stress release following deglaciation. Timing of initial rock avalanche runout deposition at one site is consistent with this conclusion, though later instability episodes at the same site may have occurred independent of the influence of paraglacial stress release