Net community production, Thorium-234 activity and particle flux during the Southern Ocean iron fertilization experiment LOHAFEX

A closed eddy core in the Subantarctic Atlantic Ocean was fertilized twice with two tons of iron (as FeSO4), and the 300 km**2 fertilized patch was studied for 39 days to test whether fertilization enhances downward particle flux into the deep ocean. Chlorophyll a and primary productivity doubled after fertilization, and photosynthetic quantum yield (FV/FM) increased from 0.33 to >0.40. Silicic acid (<2 µmol/L) limited diatoms, which contributed <10% of phytoplankton biomass. Copepods exerted high grazing pressure. This is the first study of particle flux out of an artificially fertilized bloom with very low diatom biomass. Net community production (NCP) inside the patch, estimated from O2:Ar ratios, averaged 21 mmol POC/m**2/d, probably ±20%. 234Th profiles implied constant export of ~6.3 mmol POC/m**2/d in the patch, similar to unfertilized waters. The difference between NCP and 234Th-derived export partly accumulated in the mixed layer and was partly remineralized between the mixed layer and 100 m. Neutrally buoyant sediment traps at 200 and 450 m inside and outside the patch caught mostly <1.1 mmol POC/m**2/d, predominantly of fecal origin; flux did not increase upon fertilization. Our data thus indicate intense flux attenuation between 100 and 200 m, and probably between the mixed layer and 100 m. We attribute the lack of fertilization-induced export to silicon limitation of diatoms and reprocessing of sinking particles by detritus feeders. Our data are consistent with the view that nitrate-rich but silicate-deficient waters are not poised for enhanced particle export upon iron addition.

Geochemical and mineralogical analyses of clay-rich intervals in ODP Sites 166-1006 and 166-1007

The western flank of the Great Bahama Bank, drilled during ODP Leg 166 at seven sites, represents a prograding carbonate sequence from late Oligocene to Holocene [Eberli et al., Proc. ODP Init. Reports 166 (1997)]. The signatures of the detrital input and of diagenetic alteration are evident in clay enriched intervals from the most distal Sites 1006 and 1007 in the Straits of Florida. Mineralogical and chemical investigations (XRD, TEM, SEM, ICP-MS) run on bulk rocks and on the clay fractions enable the origin and evolution of silicate parageneses to be characterized. Plio-Pleistocene silt and clay interbeds contain detrital clay assemblages comprising chlorite, illite, interstratified illite smectite, smectite, kaolinite and palygorskite. The greater smectite input within late Pliocene units than in Pleistocene oozes may relate either varying source areas or change in paleoclimatic conditions and weathering intensity. The clay intervals from Miocene-upper Oligocene wackestone sections are fairly different, with prevalent smectite in the fine fraction, whose high crystallinity and Mg contents that point towards an authigenic origin. The lower Miocene section, below 1104 mbsf, at depths where compaction features are well developed, is particularly characterized by abundant authigenic Na-K-clinoptilolite filling foraminifer tests. The authigenic smectite and clinoptilolite paragenesis is recorded by the chemical trends, both of the sediment and the interstitial fluid. This diagenetic evolution implies Si- and Mg rich fluids circulating in deeper and older sequences. For lack of any local volcaniclastic input, the genesis of zeolite and the terms of water rock interaction are discussed. The location of the diagenetic front correlates with that of the seismic sequence boundary P2 dated as 23.2 Ma. This correspondence may allow the chronostratigraphic significance of some specific seismic reflections to be reassessed.

Geochemistry on manganese nodules from Loch Fyne, Scotland

Manganese nodules and manganese carbonate concretions occur in the upper 10-15 cm of the Recent sediments of Loch Fyne, Argyllshire in water depths of 180-200 m. The nodules are spherical, a few mm to 3 cm in diameter, and consist of a black, Mn-rich core and a thin, red, Fe-rich rim. The carbonate occurs as irregular concretions, 0.5-8 cm in size, and as a cement in irregular nodule and shell fragment aggregates. It partially replaces some nodule material and clastic silicate inclusions, but does not affect aragonitic and calcitic shell fragments. The nodules are approximately 75% pure oxides and contain 30% Mn and 4% Fe. In the cores, the principal mineral phase is todorokite, with a Mn/Fe ratio of 17. The rim consists of X-ray amorphous Fe and Mn oxides with a Mn/Fe ratio of 0.66. The cores are enriched, relative to Al, in K, Ba, Co, Mo, Ni and Sr while the rims contain more P, Ti, As, Pb, Y and Zn. The manganese carbonate has the composition (Mn47.7 Ca45.1 Mg7.2) CO3. Apart from Cu, all minor elements are excluded from significant substitution in the carbonate lattice. Manganese nodules and carbonates form diagenetically within the Recent sediments of Loch Fyne. This accounts for the high Mn/Fe ratios in the oxide phases and the abundance of manganese carbonate concretions. Mn concentrations in the interstitial waters of sediment cores are high (ca. 10 ppm) as also, by inference, are the dissolved carbonate concentrations.

Chemical and Sr isotopic compositions of 3 step leachates and residues of bulk samples, zeolites, and smectites from Lower Miocene sediments at ODP Site 166-1007

Massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor (mbsf) in lower Miocene wellcompacted carbonate periplatform sediments from the Great Bahama Bank [Ocean Drilling Program, ODP Leg 166, Site 1007]. The diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix, as well as Mg smectites. In carbonate-rich deposits, the formation of the zeolite requires a supply of silica. Thus, the objective of the study is to determine the origin of the silica supply, its diagenetic evolution, and consequently the related implications on interpretation of the sedimentary record, in terms of local or global paleoceanographic change. For lack of evidence for any volcaniclastic input or traces of Si-enriched deep fluids circulation, an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow-water carbonate sequences. Geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite, as well as its contemporaneous formation with the carbonate deposition (Sr model ages of 19.6-23.2 Ma). The test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage. Seawater-rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica, which reacts with clay minerals. The amount of silica (opal-A) involved in the reaction has to be significant enough, at least 10 wt.%, to account for the observed content of clinoptilolite occurring at the most zeolite-rich level. Modeling also shows that the observed amount of clinoptilolite (~19%) reflects an in situ and short-term reaction due to the high reactivity of primary biogenic silica (opal-A) until its complete depletion. The episodic occurrence of these well-lithified zeolite-rich levels is consistent with the occurrence of seismic reflectors, particularly the P2 seismic sequence boundary located at 1115 mbsf depth and dated as 23.2 Ma. The age range of most zeolitic sedimentary levels (biostratigraphic ages of 21.5-22 Ma) correlates well with that of the early Miocene glaciation Mi-1 and Mi-1a global events. Thus, the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply, or in other sensitive marine areas, is potentially a significant new proxy for paleoproductivity and oceanic global events, such as the Miocene events, which are usually recognized in deep-sea pelagic sediments and high latitude deposits.

Lead 210 and silicate profiles from sediments of the equatorial Pacific and South Atlantic

Particle mixing rates have been determined for 5 South Atlantic/Antarctic and 3 equatorial Pacific deep-sea cores using excess 210Pb and 32Si measurements. Radionuclide profiles from these siliceous, calcareous, and clay-rich sediments have been evaluated using a steady state vertical advection diffusion model. In Antarctic siliceous sediments210Pb mixing coefficients (0.04-0.16 cm**2/y) are in reasonable agreement with the 32Si mixing coefficient (0.2 or 0.4 cm**2/y, depending on 32Si half-life). In an equatorial Pacific sediment core, however, the 210Pb mixing coefficient (0.22 cm**2/y) is 3-7 times greater than the 32Si mixing coefficient (0.03 or 0.07 cm**2/y). The difference in 210Pb and 32Si mixing rates in the Pacific sediments results from: (1) non-steady state mixing and differences in characteristic time and depth scales of the two radionuclides, (2) preferential mixing of fine-grained clay particles containing most of the 210Pb activity relative to coarser particles (large radiolaria) containing the 32Si activity, or (3) the supply of 222Rn from the bottom of manganese nodules which increases the measured excess 210Pb activity (relative to 226Ra) at depth and artificially increases the 210Pb mixing coefficient. Based on 32Si data and pore water silica profiles, dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32Si profile in the mixed layer. Deep-sea particle mixing rates reported in this study and the literature do not correlate with sediment type, sediment accumulation rate, or surface productivity. Based on differences in mixing rate among three Antarctic cores collected within 50 km of each other, local variability in the intensity of deep-sea mixing appears to be as important as regional differences in sediment properties.

Compositions of Ni-rich magnesiowüstite and coexisting magnesioferrite spinel from sediments of DSDP Holes 86-577 and 91-596

We have found trace inclusions of Ni-rich magnesiowüstite within grains of magnesioferrite spinel recovered from Cretaceous/Tertiary boundary sediments from DSDP Site 596, South Pacific (23°51.20'S, 169°39.27'W) and DSDP Site 577, North Pacific (3°6.51'N, 157°43.40'E). Measured compositions of these inclusions range from (Mg_0.85Ni_0.74Fe_0.17)O to (Mg_0.74Ni_0.09Fe_0.17)O. Coexisting magnesioferrite and magnesiowüstite can only crystallize from ultramafic, refractory, Mg-rich liquids with Mg/Si > 2 (atom ratio). Such liquid compositions cannot form as a result of fractional crystallization and are unknown to occur as a result of terrestrial igneous processes or meteoroid ablation. We infer that these minerals crystallized from liquid droplets that equilibrated with silicate vapor at high temperatures (probably >2300°C), resulting in fractionation of volatile SiO2 from more refractory MgO. The most plausible source of this high-temperature vapor is in the fireball of the major impact event that terminated the Cretaceous.

Surface time series of nitrate, silicate and chlorophyll a (1999-2011) and CTD profiles (1999) at the PhytoCly station, Bay of Calvi, Corsica, Mediterranean Sea

This work is based on a long time series of data collected in the well-preserved Bay of Calvi (Corsica island, Ligurian Sea, NW Mediterranean) between 1979 and 2011, which include physical characteristics (31 years), chlorophyll a (chl a, 15 years), and inorganic nutrients (13 years). Because samples were collected at relatively high frequencies, which ranged from daily to biweekly during the winter-spring period, it was possible to (1) evidence the key role of two interacting physical variables, i.e. water temperature and wind intensity, on nutrient replenishment and phytoplankton dynamics during the winter-spring period, (2) determine critical values of physical factors that explained interannual variability in the replenishment of surface nutrients and the winter-spring phytoplankton bloom, and (3) identify previously unrecognized characteristics of the planktonic ecosystem. Over the >30 year observation period, the main driver of nutrient replenishment and phytoplankton (chl a) development was the number of wind events (mean daily wind speed >5 m s-1) during the cold-water period (subsurface water <13.5°C). According to winter intensity, there were strong differences in both the duration and intensity of nutrient fertilization and phytoplankton blooms (chl a). The trophic character of the Bay of Calvi changed according to years, and ranged from very oligotrophic (i.e. subtropical regime, characterized by low seasonal variability) to mesotrophic (i.e. temperate regime, with a well-marked increase in nutrient concentrations and chl a during the winter-spring period) during mild and moderate winters, respectively. A third regime occurred during severe winters characterized by specific wind conditions (i.e. high frequency of northeasterly winds), when Mediterranean "high nutrient - low chlorophyll" conditions occurred as a result of enhanced crossshore exchanges and associated offshore export of the nutrient-rich water. There was no long-term trend (e.g. climatic) in either nutrient replenishment or the winter-spring phytoplankton bloom between 1979 and 2011, but both nutrients and chl a reflected interannual and decadal changes in winter intensity.

Silicate mineralogy of basalts at DSDP Leg 54 Holes

Basalts drilled from the East Pacific Rise, OCP Ridge, and Siqueiros fracture zone during Leg 54 are texturally diverse. Dolerites are equigranular at Sites 422 and 428 and porphyritic, with phenocrysts of plagioclase (An69.73) and Ca-rich clinopyroxene (Ca42Mg48Fe10) at Site 427. The East Pacific Rise lavas and some of those from the OCP Ridge are fine-grained and porphyritic. The majority of the large crystals are clustered skeletal glomerocrysts of plagioclase An64-77), together with olivine (Fo80-87), Ca-rich clinopyroxene, or both. Euhedral phenocrysts of plagioclase, together with olivine, Carich clinopyroxene, and Cr-Al spinel in some cases, occur in most of the fine-grained lavas. These phenocrysts are small (maximum dimension <1 mm in all but one sample), sparse (combined modal amount <1% in all samples), and distinctive from the megacrysts which characterize many ocean-floor lavas. In two East Pacific Rise lavas, zoned plagioclase (An83 cores) is the sole phenocryst phase. In other porphyritic lavas from all the main East Pacific Rise and OCP Ridge units drilled during Leg 54, the plagioclase phenocrysts contain cores of bytownite (An79-87) surrounded by more-sodic feldspar (An67-77). Core/rim relationships vary from continuous normal zoning, through discontinuous zoning, to extensive resorption of the calcic cores in some samples. The compositions of the plagioclase calcic cores are systematically related to those of the glomerophyric plagioclase and olivine in the lavas containing them. Furthermore, only one compositional population of calcic cores occurs in each rock. The possible causes of these relationships are far from clear. Magma mixing, although superficially applicable, is inconsistent with important aspects of the phenocryst mineralogy of these particular lavas. A more satisfactory model to explain both phenocryst zoning and rapid glomerocryst growth immediately before extrusion may be constructed by postulating influx of water into the upwelling magmas within Layer 3 of the oceanic crust beneath the East Pacific Rise, and subsequent loss of part of this water during effervescence within feeder dykes between Layer 3 and the ocean floor. It is shown that this model is fully consistent with published data on water and carbon dioxide contents and ratios in the pillow-margin glasses, vesicles, and phenocryst inclusions of ocean-floor basalts. The evidence for the precipitation of plagioclase- dominated crystalline assemblages from these magmas in the upper part of Layer 3 is concordant with recent geophysically based modeling of the structure of the East Pacific Rise. Calcium-rich clinopyroxenes in dolerites from the OCP Ridge and Siqueiros fracture zone show radial, oscillatory, and sector-zoning. In Sample 428A-5-2 (Piece 5a), the compositional trends resulting from this zoning closely resemble those of the pyroxenes in some lunar lavas. The controls on crystallization of interstitial pigeonite - epitaxial upon augite - in this rock are discussed. Both sector-zoning of the augite and nucleation of pigeonite within microvolumes of magma with a low Ca(Mg + Fe) ratio appear to be important factors.

Whole rock and Cr-rich spinel geochemistry of ODP Legs 152 and 163 and DSDP Hole 38-338 samples

Coring during Ocean Drilling Program and Deep Sea Drilling Project Legs 163, 152, 104, 81, and 38 recovered sequences of altered basalt from North Atlantic seaward-dipping reflector sequences (SDRS) erupted during the initial rifting of Greenland from northern Europe and likely associated with excessive mantle temperatures caused by an impacting mantle plume head. Cr-rich spinel is found abundantly as inclusions and groundmass crystals within the olivine-rich lavas of Hole 917A (Leg 152) cored into the Southeast Greenland SDRS, but only rarely as inclusions within plagioclase in the lavas of the Vøring Plateau SDRS, and it is absent from other cored SDRS lavas from the Rockall Plateau and Southeast Greenland. Eruptive melt compositions determined from inferred, thermodynamically-defined, spinel-melt exchange equilibria indicate that the most primitive melts represented by Hole 917A basalts have Mg/(Mg + Fe2+) at least as high as 0.70 and approach near-primary mantle melt compositions. In contrast, Cr-rich spinels from Hole 338 (Leg 38) lavas on the Vøring Plateau SDRS give evidence for melt with Mg/(Mg + Fe2+) only as high as 0.64. This study underlines that primitive melts similar to those from Hole 917A comprise only a small fraction of the eruptive North Atlantic SDRS melts, and that most SDRS basalts were, in fact, too evolved to have precipitated Cr-rich spinel, with true melt Mg/(Mg + Fe2+) likely below 0.60. The evolved nature of the SDRS basalts implies large amounts of fractionation at the base of the crust or deep within it, consistent with seismic results that indicate an abnormally thick Layer 3 underlying the SDRS.