Description: The upper 200 m of the sediments recovered during IODP Leg 302, the Arctic Coring Expedition (ACEX), to the Lomonosov Ridge in the central Arctic Ocean consist almost exclusively of detrital material. The scarcity of biostratigraphic markers severely complicates the establishment of a reliable chronostratigraphic framework for these sediments, which contain the first continuous record of the Neogene environmental and climatic evolution of the Arctic region. Here we present profiles of cosmogenic 10Be together with the seawater-derived fraction of stable 9Be obtained from the ACEX cores. The down-core decrease of 10Be/9Be provides an average sedimentation rate of 14.5 ± 1 m/Ma for the uppermost 151 m of the ACEX record and allows the establishment of a chronostratigraphy for the past 12.3 Ma. The age-corrected 10Be concentrations and 10Be/9Be ratios suggest the existence of an essentially continuous sea ice cover over the past 12.3 Ma.

Description: Time series of lead (Pb) and neodymium (Nd) isotope compositions were measured on three ferromanganese crusts recording the evolution of NE Atlantic water masses over the past 15 Ma. The crusts are distributed along a depth profile (∼700–4600 m) comprising the present-day depths of Mediterranean Outflow Water and North East Atlantic Deep Water. A pronounced increase of the 206Pb/204Pb in the two deeper crusts starting at ∼4 Ma and a decrease in 143Nd/144Nd in all three crusts took place between ∼6–4 Ma and the present. These patterns are similar to isotope time series in the western North Atlantic basin and are consistent with efficient mixing between the two basins. However, the changes occurred 1–3 Ma earlier in the eastern basin indicating that the northeastern Atlantic led the major change in Pb and Nd isotope composition, probably due to a direct supply of Labrador Seawater via a northern route. The Pb isotope evolution during the Pliocene-Pleistocene can generally be explained by mixing between two end-members corresponding to Mediterranean Outflow Water and North East Atlantic Deep Water, but external sources such as Saharan dust are likely to have played a role as well. The Pb isotope composition of the shallowest crust that grew within the present-day Mediterranean Outflow Water does not show significant Pb isotope changes indicating that it was controlled by the same Pb sources throughout the past 15 Ma.

Description: The initiation of Northern Hemisphere glaciation in the subarctic North Pacific at ∼2.73 Ma was marked by an abrupt cessation of high opaline accumulation, considered to result from an increased stratification of the water column that should have led to higher utilization of nutrients in the surface ocean. We present a new stable Si isotope-based record of Si utilization that is hard to reconcile with this model. A drop in 30Si/28Si by 0.4‰ at 2.73 Ma is coincident with an increase in bulk N isotope composition. The contrasting utilization records cannot have been both caused by a hydrographic change alone. Excluding a change in the Si:N export ratio, these results either imply a relative increase in silicic acid supplied to the surface waters or a change in its Si isotope composition. While it is impossible to distinguish between these two possibilities, both imply a regional or global change in the Si biogeochemical cycle, potentially caused by an enhanced storage of Si in the underlying deep waters of the Pacific.

Description: Cenozoic biostratigraphic, cosmogenic isotope, magnetostratigraphic, and cyclostratigraphic data derived from Integrated Ocean Drilling Program Expedition 302, the Arctic Coring Expedition (ACEX), are merged into a coherent age model. This age model has low resolution because of poor core recovery, limited availability of biostratigraphic information, and the complex nature of the magnetostratigraphic record. One 2.2 Ma long hiatus occurs in the late Miocene; another spans 26 Ma (18.2–44.4 Ma). The average sedimentation rate in the recovered Cenozoic sediments is about 15 m/Ma. Core-seismic correlation links the ACEX sediments to the reflection seismic stratigraphy of line AWI-91090, on which the ACEX sites were drilled. This seismostratigraphy can be correlated over wide geographic areas in the central Arctic Ocean, implying that the ACEX age model can be extended well beyond the drill sites.

Description: Detailed geological mapping in the Drosh-Shishi area in southern Chitral (NW Pakistan) was combined with high-precision U-Pb dating on zircons to constrain the timing of magmatism and associated deformation/metamorphic events related to the Kohistan-Karakoram convergence and collision. Our new ages indicate that the Mesozoic to Tertiary magmatic history of this region is influenced by long-lived melt generation above an active subduction zone. Dated intrusive rocks range in age from 130 to 39 million years, indicating that subduction-related magmatism continued after the Kohistan-Karakoram and the India-Asia collisions. Initial hafnium isotope ratios were measured on the dated zircons to constrain the type of melt source of the dated plutons. The data reveal the different nature of partly coeval magmatism in these units, i.e. continental arc magmatism in the Karakoram (ca. 130-104 Ma) and arc magmatism magmatism on the Kohistan side (112-39 Ma). Intrusions within the suture zone can be clearly traced to be Karakoram-derived on the basis of initial Hf isotopic compositions. Granite dykes crosscutting the Kohistan units have sampled an underlying, old continental basement of Gondwana affinity. The geochronological evidence presented in this paper is consistent with Cretaceous subduction beneath the Karakoram Terrane. The related calc-alkaline magmatism seems to have stopped at about 100 Ma. Granite dykes on the Kohistan side show that the magmatic and tectonic history of the Karakoram-Kohistan Suture Zone continued to the Eocene. This long tectono-metamorphic and magmatic activity in the arc plates was likely due to complex and few million year long interplays between subduction and thrusting events in the forearc, within-arc and back-arc regions between two active subduction zones.

Description: [1] Radiogenic isotope compositions (Sr, Nd, Pb, Hf, and Os) of sediment-hosted seafloor ferromanganese crusts and sediments incrusted with ferromanganese oxyhydroxides from the Lesser Antilles island arc were measured to distinguish between hydrogenous (seawater-derived) and hydrothermal metal sources. The ages of the precipitates range between recent (last few thousand years) and a few 100 kyr as deduced from 10Be and Co concentrations. Evidence from the presence of bladed todorokite and nontronite, together with the major element and REE composition, suggests that a significant proportion of these sediment-hosted precipitates formed at relatively low temperatures from a mixture of seawater and hydrothermal fluids associated with island arc volcanism. The radiogenic isotope compositions of all metals mentioned above, except Pb, show large differences in hydrothermal versus hydrogenous contributions over space and time. In contrast to precipitates of high-temperature fluids which mainly scavenge their REE contents from seawater the crusts of this study show 143Nd/144Nd of up to 0.512817 (ɛNd = +3.5). This is close to the signature of the nearby island arc rocks and far above the expected local seawater ratio of ∼0.51209 (ɛNd = −10.7). These crusts also show high 176Hf/177Hf (up to 0.283102), low 87Sr/86Sr (up to 0.7069), and low 187Os/188Os (up to 0.16) compared with local seawater, as expected from hydrothermal, island-arc-derived metal contributions. In contrast, the Pb isotope signatures of the crusts cannot be explained by mixing between seawater and hydrothermal sources. It is suggested that Pb was either removed from the ascending fluids within the sediment column before they reached seawater or the temperatures were too low to leach significant amounts of Pb from the rocks or sediments. External sources such as Saharan dust, particulate inputs from the Orinoco River, or even incongruent release of Pb isotopes from the island arc rock-derived particles must have contributed to the observed Pb isotope variability. Our results suggest that submarine hydrothermalism originating from intraoceanic island arc volcanism creates distinct geochemical environments for the dispersion of hydrothermal fluids and may be an important mechanism to supply metals of hydrothermal origin to seawater.

Description: Lead (Pb), neodymium (Nd), and strontium (Sr) isotopic analyses were carried out on sediment leachates (reflecting the isotope composition of past seawater) and digests of the bulk residues (reflecting detrital continental inputs) of Integrated Ocean Drilling Program (IODP) Leg 302 and core PS2185 from the Lomonosov Ridge (Arctic Ocean). Our records are interpreted to reflect changes in continental erosion and oceanic circulation, driven predominantly by tectonic forcing on million-year timescales in the older (pre-2 Ma) part of the record and by climatic forcing of weathering and erosion of the Eurasian continental margin on thousand-year timescales in the younger (post-2 Ma) part. These data, covering the past ∼15 Ma, show that continental inputs to the central Arctic Ocean have been more closely linked to glacial and hydrological processes occurring on the Eurasian margin than on continental North America and Greenland. The constancy of the detrital input signatures supports the early existence of an Arctic sea ice cover, whereas the major initiation of Northern Hemisphere glaciation at 2.7 Ma appears to have had little impact on the weathering regime of the Eurasian continental margin.

Description: Down-core variations in North Atlantic 231Paxs/230Thxs have been interpreted as changes in the strength of the Atlantic meridional overturning circulation (AMOC). This modeling study confirms that hypothetical changes in the AMOC would indeed be recorded as changes in the distribution of sedimentary 231Paxs/230Thxs. At different sites in the North Atlantic the changes in sedimentary 231Pa/230Th that we simulate are diverse and do not reflect a simple tendency for 231Paxs/230Thxs to increase toward the production ratio (0.093) when the AMOC strength reduces but instead are moderated by the particle flux. In its collapsed or reduced state the AMOC does not remove 231Pa from the North Atlantic: Instead, 231Pa is scavenged to the North Atlantic sediment in areas of high particle flux. In this way the North Atlantic 231Paxs/230Thxs during AMOC shutdown follows the same pattern as 231Paxs/230Thxs in modern ocean basins with reduced rates of meridional overturning (i.e., Pacific or Indian oceans). We suggest that mapping the spatial distribution of 231Paxs/230Thxs across several key points in the North Atlantic is an achievable and practical qualitative indicator of the AMOC strength in the short term. Our results indicate that additional North Atlantic sites where down-core observations of 231Paxs/230Thxs would be useful coincide with locations which were maxima in the vertical particle flux during these periods. Reliable estimates of the North Atlantic mean 231Paxs/230Thxs should remain a goal in the longer term. Our results hint at a possible ‘‘seesaw-like’’ behavior in 231Pa/230Th in the South Atlantic.