Dissolved iron(II) in the Pacific Ocean: Measurements from the PO2 and P16N CLIVAR/CO2 repeat hydrography expeditions

The redox speciation of dissolved iron in seawater was evaluated at 121 locations in the Pacific Ocean at depths of 15-1000 m, using the method of luminol chemiluminescence. The results indicate that reduced iron, Fe(II), is ubiquitous in surface seawater with a relatively consistent pattern of occurrence. Surface maxima were present in most profiles, with median concentrations of 25-30 pM representing 12-14% of the total dissolved iron. Concentrations decreased monotonically with depth to<12 pM within the upper euphotic zone. This pattern was observed during both day and nighttime sampling events, which suggests that non-photochemical production mechanisms can produce photochemical-like signatures. Further, if theoretical rates of Fe(II) oxidation are applicable to the open ocean, then the employed sampling methods precluded assessment of photochemically-produced Fe(II), regardless of ambient light conditions. For this and other reasons, the concentrations reported here for the upper water column likely represent lower limits of labile iron concentration, and suggest that dissolved iron may be more available for uptake than previously believed. Deeper in the water column, Fe(II) was also frequently detected, though it constituted a small fraction of the total dissolved iron. Possible source mechanisms at these depths include thermal (dark) reduction of Fe(III) organic complexes or remineralization of sinking biogenic particles containing Fe(II). In the northern Philippine Sea between the Japanese coast and the Izu-Bonin volcanic arc system, Fe(II) concentrations were found to be atypically high, possibly because of high atmospheric dust deposition near the surface and transport of sediment-derived iron at depth.     

Deep-water Circulation: Processes & Products (16�C18 June 2010, Baiona): introduction and future challenges

Deep-water circulation is a critical part of the global conveyor belt that regulates Earth??s climate. The bottom (contour)-current component of this circulation is of key significance in shaping the deep seafloor through erosion, transport, and deposition. As a result, there exists a high variety of large-scale erosional and depositional features (drifts) that together form more complex contourite depositional systems on continental slopes and rises as well as in ocean basins, generated by different water masses flowing at different depths and at different speeds either in the same or in opposite directions. Yet, the nature of these deep-water processes and the deposited contourites is still poorly understood in detail. Their ultimate decoding will undoubtedly yield information of fundamental importance to the earth and ocean sciences. The international congress Deep-water Circulation: Processes & Products was held from 16?C18 June 2010 in Baiona, Spain, hosted by the University of Vigo. Volume 31(5/6) of Geo-Marine Letters is a special double issue containing 17 selected contributions from the congress, guest edited by F.J. Hernández-Molina, D.A.V. Stow, E. Llave, M. Rebesco, G. Ercilla, D. Van Rooij, A. Mena, J.-T. Vázquez and A.H.L. Voelker. The papers and discussions at the congress and the articles in this special issue provide a truly multidisciplinary perspective of interest to both academic and industrial participants, contributing to the advancement of knowledge on deep-water bottom circulation and related processes, as well as contourite sedimentation. The multidisciplinary contributions (including geomorphology, tectonics, stratigraphy, sedimentology, paleoceanography, physical oceanography, and deep-water ecology) have demonstrated that advances in paleoceanographic reconstructions and our understanding of the ocean??s role in the global climate system depend largely on the feedbacks among disciplines. New insights into the link between the biota of deep-water ecosystems and bottom currents confirm the need for this field to be investigated and mapped in detail. Likewise, it is confirmed that deep-water contourites are not only of academic interest but also potential resources of economic value. Cumulatively, both the congress and the present volume serve to demonstrate that the role of bottom currents in shaping the seafloor has to date been generally underestimated, and that our understanding of such systems is still in its infancy. Future research on contourites, using new and more advanced techniques, should focus on a more detailed visualization of water-mass circulation and its variability, in order to decipher the physical processes involved and the associations between drifts and other common bedforms. Moreover, contourite facies models should be better established, including their associations with other deep-water sedimentary environments both in modern and ancient submarine domains. The rapid increase in deep-water exploration and the new deep-water technologies available to the oil industry and academic institutions will undoubtedly lead to spectacular advances in contourite research in terms of processes, morphology, sediment stacking patterns, facies, and their relationships with other deep-marine depositional systems.     

Paleoshorelines and lowstand sedimentation on subtropical shelves: a case study from the Fraser Shelf,Australia

Evidence of submerged paleoshorelines on tectonically stable continental shelves is represented by relict coastal depositional features that formed during periods of lower sea level. This study investigates two paleoshoreline features that extend 70?km from the Barwon Bank along the southeast Fraser Shelf, off eastern Australia. They are defined by well-lithified, mixed carbonate-siliciclastic deposits now at water depths of 60?m and between 80 and 100?m, located approximately 40?km seaward of the modern shoreline. High-resolution multibeam bathymetry data show geomorphic features that we interpret as paleodunes in the case of the 60?m feature, and paleobarriers in the case of the 80–100?m feature. Sub-bottom profiles also show evidence of prograded beach foresets and paleochannels as components of the Barwon Bank, which is a low and elongate bank that rises about 30?m above the surrounding shelf to a depth of 60?m. Carbonate cements in the rocks forming the paleoshoreline features reveal diagenetic influence of meteoric waters, suggesting that subaerial processes contributed to the formation and preservation of both features. Radiocarbon dating of bioclastic grains (coralline algae, benthic forams) yields an age range between 22.8 and 20.4?ka for the paleobarrier, and an age of 17.3?ka for the paleodune. The morphology of the submerged paleodunes is analogous to modern parabolic dunes on nearby Fraser and Moreton islands. The position and morphology of the paleodunes provide proxy information about the climate history of Australia during the late Quaternary, as these dunes are a likely consequence of the general continent-wide aridity during the Last Glacial Maximum. The preservation of the eolian dunes during marine transgression, despite their direct exposure to coastal hydrodynamic processes, suggests that the dunes were armoured, stabilised and lithified during the glacial lowstand, prior to drowning.     

Sediment instability on the Portuguese continental margin under abrupt glacial climate changes (last 60 kyr)

It is well established that orbital scale sea-level changes generated larger transport of sediments into the deep-sea during the last glacial maximum than the Holocene. However, the response of sedimentary processes to abrupt millennial-scale climate variability is rather unknown. Frequency of distal turbidites and amounts of advected detrital carbonate are estimated off the Lisbon–Setúbal canyons (core MD03-2698, at 4602 mwd), within a chronostratigraphy based on radiometric ages, oxygen isotopes and paleomagnetic key global anomalies. We found that: 1) higher frequency of turbidites concurred with Northern Hemisphere coldest temperatures (Greenland Stadials [GS], including Heinrich [H] events). But more than that, an escalating frequency of turbidites starts with the onset of global sea-level rising (and warming in Antarctica) and culminates during H events, at the time when rising is still in its early-mid stage, and the Atlantic Meridional Overturning Circulation (AMOC) is re-starting. This short time span coincides with maximum gradients of ocean surface and bottom temperatures between GS and Antarctic warmings (Antarctic Isotope Maximum; AIM 17, 14, 12, 8, 4, 2) and rapid sea-level rises. 2) Trigger of turbidity currents is not the only sedimentary process responding to millennial variability; land-detrital carbonate (with a very negative bulk δ¹⁸O signature) enters the deep-sea by density-driven slope lateral advection, accordingly during GS. 3) Possible mechanisms to create slope instability on the Portuguese continental margin are sea-level variations as small as 20 m, and slope friction by rapid deep and intermediate re-accommodation of water masses circulation. 4) Common forcing mechanisms appear to drive slope instability at both millennial and orbital scales.     

Estimates of wind power and radiative near-inertial internal wave flux

Ocean Dynamics - Energy transfer mechanisms between the atmosphere and the deep ocean have been studied for many years. Their importance to the ocean’s energy balance and possible...     

The Gulf of Cadiz: an unstable giant contouritic levee

Recent multibeam bathymetry and acoustic imagery data provide a new understanding of the sedimentary system located in the Gulf of Cadiz which is under the influence of a strong current, the Mediterranean Outflow Water (MOW). When it comes out from the Strait of Gibraltar, the MOW is either channelled along major or secondary channels, or spills over a sedimentary levee. Frequent earthquakes and the constant current shearing generate widespread sediment deformation and instability of contourite deposits. Secondary channels can form by retrogression following an initial failure. At their mouth, sediment accumulates in the form of small sandy contourite lobes. These observations suggest that the Gulf of Cadiz system shares many similarities with channel–levee complexes formed by turbidity current activity. The main difference is that, in the Gulf of Cadiz, the main process is a strongly flowing saline current which locally interacts with gravity processes.