Benthic respiration and physical oceanography on two contrasting continental margins in the western Indian Ocean: the Yemen-Somali upwelling region and the margin off Kenya

During the Netherlands Indian Ocean Project (NIOP, 1992-1993) sediment community oxygen consumption (SCOC) was measured on two continental margins in the Indian Ocean with different productivity: the productive upwelling region off Yemen-Somalia and the supposedly less productive Kenyan margin, which lacks upwelling. The two margins also differ in terms of river input (Kenya) and the more severe oxygen minimum in the Arabian Sea. Simultaneously with SCOC, distributions of benthic biomass and phytodetritus were studied. Our expectation was that benthic processes in the upwelling margin of the Arabian Sea would be relatively enhanced as a result of the higher productivity. On the Kenyan margin, SCOC (range 1-36 mmol/m**2/d) showed a clear decrease with increasing water depth, and little temporal variation was detected between June and December. Highest SCOC values of this study were recorded at 50 m depth off Kenya, with a maximum of 36 mmol/m**2/d in the northernmost part. On the margin off Yemen-Somalia, SCOC was on average lower and showed little downslope variation, 1.8-5.7 mmol/m**2/d, notably during upwelling, when the zone between 70 and 1700 m was covered with low O2 water (10-50 µM). After cessation of upwelling, SCOC at 60 m depth off Yemen increased from 5.7 to 17.6 mmol/m**2/d concurrently with an increase of the near-bottom O2 concentration (from 11 to 153 µM), suggesting a close coupling between SCOC and O2 concentration. This was demonstrated in shipboard cores in which the O2 concentration in the overlying water was raised after the cores were first incubated under in situ conditions (17 µM O2). This induced an immediate and pronounced increase of SCOC. Conversely, at deeper stations permanently within the oxygen minimum zone (OMZ), SCOC showed little variation between monsoon periods. Hence, organic carbon degradation in sediments on a large part of the Yemen slope appears hampered by the oxygen deficiency of the overlying water. Macrofauna biomass and the pooled biomass of smaller organisms, estimated by the nucleic acid content of the sediment, had comparable ranges in the two areas in spite of more severe suboxic conditions in the Arabian Sea. At the Kenyan shelf, benthic fauna (macro- and meiofauna) largely followed the spatial pattern of SCOC, i.e. high values on the northern shelf-upper slope and a downslope decrease. On the Yemen-Somali margin the macrofauna distribution was more erratic. Nucleic acids displayed no clear downslope trend on either margin owing to depressed values in the OMZ, perhaps because of adverse effects of low O2 on small organisms (meiofauna and microbes). Phytodetritus distributions were different on the two margins. Whereas pigment levels decreased downslope along the Kenya margin, the upper slope off Yemen (800 m) had a distinct accumulation of mainly refractory carotenoid pigments, suggesting preservation under low 02. Because the accumulations of Corg and pigments on the Yemen slope overlap only partly, we infer a selective deposition and preservation of labile particles on the upper slope, whereas refractory material undergoes further transport downslope.

Composition of phosphorites from continental margins, seamounts, and islands

Behavior of molybdenum and manganese is studied in phosphorite samples from shelves, seamounts, and islands of the ocean. In shelf phosphorites molybdenum and manganese contents are 2-128 and 12-1915 ppm, respectively, while the Mo/Mn ratio varies from 0.004 to 4.5. Phosphorites from ocean seamounts impregnated with ferromanganese oxyhydroxides contain 0.84-14.5 ppm Mo and 0.1-17% Mn. The Mo/Mn ratio varies within 0.0008-0.004. Phosphate bearing ferromanganese crusts overlying seamount phosphorites contain 54-798 ppm Mo and 10-20% Mn; the Mo/Mn ratio varies within 0.002-0.005. Corresponding values for most island phosphorites are 0.44-11.2 ppm, 27-287 ppm, and 0.008-0.20. Phosphorites from reduced environment are characterized by relative enrichment in Mo and depletion in Mn, whereas the Mo/Mn ratio reaches maximum values. The ratio decreases with transition to suboxic and oxic conditions. Molybdenum content in recent shelf sediments is commonly higher than that in authigenic phosphorites from these sediments. Recent phosphorite nodules from the Namibian shelf become depleted in Mo and Mn during their lithification, but Pliocene-Pleistocene nodules of similar composition and origin from the same region are enriched in Mo and characterized by variable Mn content. Higher Mo contents in phosphate bearing ferromanganese crusts result from coprecipitation of Mo and Mn from seawater. Unweathered phosphorites on continents and phosphorites from ocean shelves are largely enriched in Mo with the Mo/Mn ratio varying from 0.01 to 1.0. This is an evidence of their formation in reduced conditions.

Age tie points and geochemistry of sediment cores from the Antarctic continental margin

Few high-latitude terrestrial records document the timing and nature of the Cenozoic "Greenhouse" to "Icehouse" transition. Here we exploit the bulk geochemistry of marine siliciclastic sediments from drill cores on Antarctica's continental margin to extract a unique semiquantitative temperature and precipitation record for Eocene to mid-Miocene (~54-13 Ma). Alkaline elements are strongly enriched in the detrital mineral fraction in fine-grained siliciclastic marine sediments and only occur as trace metals in the biogenic fraction. Hence, terrestrial climofunctions similar to the chemical index of alteration (CIA) can be applied to the alkaline major element geochemistry of marine sediments on continental margins in order to reconstruct changes in precipitation and temperature. We validate this approach by comparison with published paleotemperature and precipitation records derived from fossil wood, leaves, and pollen and find remarkable agreement, despite uncertainties in the calibrations of the different proxies. A long-term cooling on the order of >=8°C is observed between the Early Eocene Climatic Optimum (~54-52 Ma) and the middle Miocene (~15-13 Ma) with the onset of transient cooling episodes in the middle Eocene at ~46-45 Ma. High-latitude stratigraphic records currently exhibit insufficient temporal resolution to reconstruct continental aridity and inferred ice-sheet development during the middle to late Eocene (~45-37 Ma). However, we find an abrupt aridification of East Antarctica near the Eocene-Oligocene transition (~34 Ma), which suggests that ice coverage influenced high-latitude atmospheric circulation patterns through albedo effects from the earliest Oligocene onward.

Organic matter preservation in sediments of the Peru and Oman continental margin

Detailed petrographical and bulk geochemical investigations of organic matter (OM) have been performed on sediments deposited below or close to upwelling areas offshore Peru (ODP-Leg 112; Sites 679, 681, 688) and Oman (ODP-Leg 117; Sites 720, 723, 724) in order to obtain a quantitative understanding of its accumulation and degradation. Microscopical as well as nanoscopical investigations reveal that the OM in sediments affected by upwelling mechanisms mainly (up to 98%) consists of unstructured (amorphous) organic aggregates without any apparent biological structures. In sediments which are not or to a lesser extent affected by upwelling (Site 720) terrestrial OM predominates. Organic carbon (TOC) contents are highly variable and range between 9.8% in sediments deposited below upwelling cells and 0.2% in sediments outside the upwelling zone. The TOC/sulphur ratios of the sediments scatter widely. The samples from the deep-water locations (Sites 688 and 720), show C/S-ratios of "normal" marine sediments, whereas at the other locations no correlation or even a negative correlation between sulphur and TOC concentration exists. In most of the upwelling-influenced sediments OM contains a significant amount of sulphur. The incorporation of sulphur into the OM followed microbial sulphate reduction and occurred in the upper meters of the sedimentary column. Below, OM is still present in vast amounts and relatively hydrogen-rich, but is nevertheless non-metabolizable and becomes the limiting factor for bacterial sulphate reduction. According to mass balance calculations 90-99% of the OM produced in the photic zone was remineralized and 1-3% was consumed by microbial sulphate reduction. The aerobic and anaerobic processes have greatly affected degradation and conservation of OM.

Occurrence (presence-only) of living Lophelia pertusa reefs in the Irish continental margin

The present data set was used as a training set for a Habitat Suitability Model. It contains occurrence (presence-only) of living Lophelia pertusa reefs in the Irish continental margin, which were assembled from databases, cruise reports and publications. A total of 4423 records were inspected and quality assessed to ensure that they (1) represented confirmed living L. pertusa reefs (so excluding 2900 records of dead and isolated coral colony records); (2) were derived from sampling equipment that allows for accurate (<200 m) geo-referencing (so excluding 620 records derived mainly from trawling and dredging activities); and (3) were not duplicated. A total of 245 occurrences were retained for the analysis. Coral observations are highly clustered in regions targeted by research expeditions, which might lead to falsely inflated model evaluation measures (Veloz, 2009). Therefore, we coarsened the distribution data by deleting all but one record within grid cells of 0.02° resolution (Davies & Guinotte 2011). The remaining 53 points were subject to a spatial cross-validation process: a random presence point was chosen, grouped with its 12 closest neighbour presence points based on Euclidean distance and withheld from model training. This process was repeated for all records, resulting in 53 replicates of spatially non-overlapping sets of test (n=13) and training (n=40) data. The final 53 occurrence records were used for model training.

Sediment and pore water data of five cores from the continental slope off Uruguay

Assessing frequency and extent of mass movement at continental margins is crucial to evaluate risks for offshore constructions and coastal areas. A multidisciplinary approach including geophysical, sedimentological, geotechnical, and geochemical methods was applied to investigate multistage mass transport deposits (MTDs) off Uruguay, on top of which no surficial hemipelagic drape was detected based on echosounder data. Nonsteady state pore water conditions are evidenced by a distinct gradient change in the sulfate (SO4**2-) profile at 2.8 m depth. A sharp sedimentological contact at 2.43 m coincides with an abrupt downward increase in shear strength from approx. 10 to >20 kPa. This boundary is interpreted as a paleosurface (and top of an older MTD) that has recently been covered by a sediment package during a younger landslide event. This youngest MTD supposedly originated from an upslope position and carried its initial pore water signature downward. The kink in the SO4**2- profile approx. 35 cm below the sedimentological and geotechnical contact indicates that bioirrigation affected the paleosurface before deposition of the youngest MTD. Based on modeling of the diffusive re-equilibration of SO4**2- the age of the most recent MTD is estimated to be <30 years. The mass movement was possibly related to an earthquake in 1988 (approx. 70 km southwest of the core location). Probabilistic slope stability back analysis of general landslide structures in the study area reveals that slope failure initiation requires additional ground accelerations. Therefore, we consider the earthquake as a reasonable trigger if additional weakening processes (e.g., erosion by previous retrogressive failure events or excess pore pressures) preconditioned the slope for failure. Our study reveals the necessity of multidisciplinary approaches to accurately recognize and date recent slope failures in complex settings such as the investigated area.

Compilation of organic carbon distribution and sedimentology in the surface sediments on the continental margin offshore southwestern Africa

In this study we demonstrate the relevance of lateral particle transport in nepheloid layers for organic carbon (OC) accumulation and burial across high-productive continental margins. We present geochemical data from surface sediments and suspended particles in the bottom nepheloid layer (BNL) from the most productive coastal upwelling area of the modern ocean, the Benguela upwelling system offshore southwest Africa. Interpretation of depositional patterns and comparison of downslope trends in OC content, organic matter composition, and 14C age between suspended particles and surface sediments indicate that lateral particle transport is the primary mechanism controlling supply and burial of OC. We propose that effective seaward particle transport primarily along the BNL is a key process that promotes and maintains local high sedimentation rates, ultimately causing high preservation of OC in a depocenter on the upper slope offshore Namibia. As lateral transport efficiently displaces areas of enhanced OC burial from maximum production at highly productive continental margins, vertical particle flux models do not sufficiently explain the relationship between primary production and shallow-marine OC burial. On geologic time scales, the widest distribution and strongest intensity of lateral particle transport is expected during periods of rapid sea-level change. At times in the geologic past, widespread downslope lateral transport of OC thus may have been a primary driver of enhanced OC burial at deeper continental slopes and abyssal basins.

Major and trace element analyses of ODP Leg 120 volcanic rocks (Table 1)

Widespread Lower Cretaceous magmatism occurred along the Indian-Australian/Antarctic margins, and in the juvenile Indian Ocean, during the rifting of eastern Gondwana. The formation of this magmatic province probably began around 120-130 Ma with the eruption of basalts on the Naturaliste Plateau and at Bunbury, western Australia. On the northeast margin of India, activity began around 117 Ma with the Rajmahal continental basalts and associated lamprophyre intrusions. The formation of the Kerguelen Plateau in the Indian Ocean began no later than 114 Ma. Ultramafic lamprophyres (alnoites) were emplaced in the Prince Charles Mountains near the Antarctic continental margin at ~ 110 Ma. These events are considered to be related to a major mantle plume, the remnant of which is situated beneath the region of Kerguelen and Heard islands at the present day. Geochemical data are presented for each of these volcanic suites and are indicative of complex interactions between asthenosphere-derived magmas and the continental lithosphere. Kerguelen Plateau basalts have Sr and Nd isotopic compositions lying outside the field for Indian Ocean mid-ocean ridge basalts (MORB) but, with the exception of Site 738 at the southern end of the plateau, within the range of more recent hotspot basalts from Kerguelen and Heard Islands. However, a number of the plateau tholeiites are characterized by lower 206Pb/204Pb ratios than are basalts from Kerguelen Island, and many also have anomalously high La/Nb ratios. These features suggest that the source of the Kerguelen Plateau basalts suffered contamination by components derived from the Gondwana continental lithosphere. An extreme expression of this lithospheric signature is shown by a tholeiite from Site 738, suggesting that the southernmost part of the Kerguelen Plateau may be underlain by continental crust. The Rajmahal tholeiites mostly fall into two distinct geochemical groups. Some Group I tholeiites have Sr and Nd isotopic compositions and incompatible element abundances, similar to Kerguelen Plateau tholeiites from Sites 749 and 750, indicating that the Kerguelen-Heard mantle plume may have directly furnished Rajmahal volcanism. However, their elevated 207Pb/204Pb ratios indicate that these magmas did not totally escape contamination by continental lithosphere. In contrast to the Group I tholeiites, significant contamination is suggested for Group II Rajmahal tholeiites, on the basis of incompatible element abundances and isotopic compositions. The Naturaliste Plateau and the Bunbury Basalt samples show varying degrees of enrichment in incompatible elements over normal MORB. The Naturaliste Plateau samples (and Bunbury Basalt) have high La/Nb ratios, a feature not inconsistent with the notion that the plateau may consist of stretched continental lithosphere, near the ocean-continent divide.

Major and trace element compositions of the Buendnerschiefer samples

The Bündnerschiefer of the Swiss-Italian Alps is a large sedimentary complex deposited on the Piemonte-Liguria and Valais oceans and associated continental margins from the upper Jurassic to Eocene. It is made of a large variety of sequences associated or not with an ophiolitic basement. The Bündnerschiefer makes an accretionary prism that developed syn-tectonically from the onset of alpine subduction, and it records orogenic metamorphism following episodes of HP metamorphism. The Bündnerschiefer shares important similarities with the Otago schists of New Zealand and with the Wepawaug schists of Connecticut, both of which form accretionary prisms and have an orogenic metamorphic imprint. With the aim of testing the hypothesis of mobility of chemical components as a function of metamorphic grade, in this work I present fifty-five bulk chemical analyses of various lithological facies of the Bündnerschiefer collected along the well-studied field gradient of the Lepontine dome of Central Switzerland, in the Prättigau half window of East Switzerland, and in the Tsaté Nappe of Valle d'Aosta (Italy). The dataset includes the concentration of major components, large ion lithophile elements (Rb, Sr, Ba, Cs), high field strength elements (Zr, Ti, Nb, Th, U, Ta, Hf), fluid-mobile light elements (B, Li), volatiles (CO2, S), REEs, and Y, V, Cr, Co, Sn, Pb, Cu, Zn, Tl, Sb, Be, and Au. These data are compared against the compositions of the global marine sediment reservoir, typical crustal reservoirs, and against the previously measured compositions of Otago and Wepawaug schists. Results reveal that, irrespective of their metamorphic evolution, the bulk chemical compositions of orogenic metasediments are characterized by mostly constant compositional ratios (e.g., K2O/Al2O3, Ba/Al2O3, Sr/CaO, etc.), whose values in most cases are undistinguishable from those of actual marine sediments and other crustal reservoirs. For these rocks, only volatile concentrations decrease dramatically as a function of metamorphic temperature, and significant deviations from the reservoir signatures are evident for SiO2, B, and Li. These results are interpreted as an indication of residual enrichment in the sediments, a process taking place during syn-metamorphic dehydration from the onset of metamorphism in a regime of chemical immobility. Residual enrichment increased the absolute concentrations of the chemical components of these rocks, but did not modify significantly their fundamental ratios. This poor compositional modification of the sediments indicates that orogenic metamorphism in general does not promote significant mass transfer from accretionary prisms. In contrast, mass transfer calculations carried out in a shear zone crosscutting the Bündnerschiefer shows that significant mass transfer occurs within these narrow zones, resulting in gains of H2O, SiO2, Al2O3, K2O, Ba, Y, Rb, Cu, V, Tl, Mo, and Ce during deformation and loss of Na2O, CO2, S, Ni, B, U, and Pb from the rock. These components were presumably transported by an aquo-carbonic fluid along the shear zone. These distinct attitudes to mobilize chemical elements from orogenic sediments may have implications for a potentially large number of geochemical processes in active continental margins, from the recycling of chemical components at plate margins to the genesis of hydrothermal ore deposits.

Radionuclides measured on water bottle samples during SONNE cruise SO202 (INOPEX)

Boundary scavenging, or the enhanced removal of adsorption-prone elements from the ocean in areas of high particle flux, is an often cited, though not well-quantified, concept used to understand the oceanic distribution of many trace metals. Because 230Th and 231Pa are produced uniformly from uranium decay and removed differentially by scavenging, the process of boundary scavenging can be elucidated by a more detailed knowledge of their water column distributions. To this end, filtered seawater was collected across the gradients in particle flux which span the subarctic Pacific: in the west during the Innovative North Pacific Experiment (INOPEX) and in the east along Line P. Lateral concentration gradients of dissolved 230Th are small throughout the subarctic Pacific at 12 sites of variable particle flux. This contradicts the prediction of the traditional boundary scavenging model. A compilation of water column data from throughout the North Pacific reveals much larger lateral concentration gradients for 230Th between the subarctic North Pacific and subtropical gyre, over lateral gradients in scavenging intensity similar to those found within the subarctic. This reflects a biogeochemical-province aspect to scavenging. Upper water column distributions of 231Pa and 231Pa/230Th ratio are consistent with the influence of scavenging by biogenic opal, while deep waters (>2.5 km) reveal an additional 231Pa sink possibly related to manganese oxides produced at continental margins or ridge crests.

Age models and stable isotope ratios from the Mid-Miocene

Mid-Miocene pelagic sedimentary sections can be correlated using intermediate and high resolution oxygen and carbon isotopic records of benthic foraminifera. Precision of a few tens of thousands of years is readily achievable at sites with high sedimentation rates, for example, Deep Sea Drilling Project sites 289 and 574. The mid-Miocene carbon isotope records are characterized by an interval of high d13C values between 17 and 13.5 Ma (the Monterey Excursion of Vincent and Berger 1985) upon which are superimposed a series of periodic or quasi-periodic fluctuations in d13C values. These fluctuations have a period of approximately 440 kyr, suggestive of the 413 kyr cycle predicted by Milankovitch theory. Vincent and Berger proposed that the Monterey Excursion was the result of increased organic carbon burial in continental margins sediments. The increased d13C values (called 13C maxima) superimposed on the generally high mid-Miocene signal coincide with increases in d18O values suggesting that periods of cooling and/or ice buildup were associated with exceptionally rapid burial of organic carbon and lowered atmospheric CO2 levels. It is likely that during the Monterey Excursion the ocean/atmosphere system became progressively more sensitive to small changes in insolation, ultimately leading to major cooling of deep water and expansion of continental ice. We have assigned an absolute chronology, based on biostratigraphic and magneto-biostratigraphic datum levels, to the isotope stratigraphy and have used that chronology to correlate unconformities, seismic reflectors, carbonate minima, and dissolution intervals. Intervals of sediment containing 13C maxima are usually better preserved than the overlying and underlying sediments, indicating that the d13C values of TCO2 in deep water and the corrosiveness of seawater are inversely correlated. This again suggests that the 13C maxima were associated with rapid burial of organic carbon and reduced levels of atmospheric CO2. The absolute chronology we have assigned to the isotopic record indicates that the major mid-Miocene deepwater cooling/ice volume expansion took 2 m.y. and was not abrupt as had been reported previously. The cooling appears abrupt at many sites because the interval is characterized by a number of dissolution intervals. The cooling was not monotonic, and the 2 m.y. interval included an episode of especially rapid cooling as well as a brief return to warmer conditions before the final phase of the cooling period. The increase in d18O values of benthic foraminifera between 14.9 and 12.9 Ma was greatest at deeper water sites and at sites closest to Antarctica. The data suggest that the d18O value of seawater increased by no more than about 1.1 per mil during this interval and that the remainder of the change in benthic d18O values resulted from cooling in Antarctic regions of deepwater formation. Equatorial planktonic foraminifera from sites 237 and 289 exhibit a series of 0.4 per mil steplike increases in d13C values. Only one of these increases in planktonic d13C is correlated with any of the features in the mid-Miocene benthic carbon isotope record.

Geochemistry at DSDP Legs 36 and 71 Holes

For Middle Jurassic to Pleistocene times, clay mineralogical and geochemical data provide information on the evolution of continental and marine paleoenvironments. They are a source of information on marginal instability, on the continental and shallow marine environments related to the development of the Southern Ocean during the Middle and Late Jurassic, and on tectonic relaxation of the continental margins at the end of the Late Jurassic. They also provide evidence for the influences of the South Atlantic opening and the movement of the Falkland Plateau in a reduced marine environment until Aptian-Albian times, and the transition to an open marine environment during Albian time; the influences of the Albian-Turonian and Coniacian-Santonian Andean deformations in an open marine environment; the limited tectonic effects and strong influence of marine currents at the Cretaceous/Tertiary boundary; the influences of the global climatic cooling and inferred bottom water circulation during the late Eocene and Oligocene; the widening of the South Atlantic Ocean during Oligocene time, which was accompanied by an increased influence of the biogenic components on sedimentation; increased carbonate dissolution from late Oligocene to early Miocene, related to the deepening of the ocean; limited mineralogical and important geochemical modifications when the Drake Passage opened in the early Miocene; the influence of the late Miocene development of the Antarctic ice-sheet; the major Antarctic cooling and Patagonian glaciation during Pliocene time; and the change in the Antarctic Bottom Water circulation at the Pliocene/Pleistocene boundary.