The phosphorus budget of the prehuman modern ocean is constrained applying the most recent estimates of the natural riverine, eolian, and ice‐rafted input fluxes; the phosphorus burial in marine ...sediments; and the hydrothermal removal of dissolved phosphate from the deep ocean. This review of current flux estimates indicates that the phosphorus budget of the ocean is unbalanced since the accumulation of phosphorus in marine sediments and altered oceanic crust exceeds the continental input of particulate and dissolved phosphorus. The phosphorus mass balance is further tested considering the dissolved phosphate distribution in the deep water column, the marine export production of particulate organic matter, rain rates of phosphorus to the seafloor, benthic dissolved phosphate fluxes, and the organic carbon to phosphorus ratios in marine particles. These independent data confirm that the phosphate and phosphorus budgets were not at steady state in the prehuman global ocean. The ocean is losing dissolved phosphate at a rate of ≥11.6 × 1010 mol yr−1 corresponding to a decline in the phosphate inventory of ≥4.5% kyr−1. Benthic data show that phosphate is preferentially retained in pelagic deep‐sea sediments where extended oxygen exposure times favor the degradation of particulate organic matter and the uptake of phosphate in manganese and iron oxides and hydroxides. Enhanced C: P regeneration ratios observed in the deep water column (>400 m water depth) probably reflect the preferential burial of phosphorus in pelagic sediments. Excess phosphate is released from continental margin sediments deposited in low‐oxygen environments. The dissolved oxygen threshold value for the enhanced release of dissolved phosphate is ∼20 μM. Benthic phosphate fluxes increase drastically when oxygen concentrations fall below this value.
Pore water and solid phase data for redox-sensitive metals (Mn, Fe, V, Mo and U) were collected on a transect across the Peru upwelling area (11°S) at water depths between 78 and 2025
m and bottom ...water oxygen concentrations ranging from ∼0 to 93
μM. By comparing authigenic mass accumulation rates and diffusive benthic fluxes, we evaluate the respective mechanisms of trace metal accumulation, retention and remobilization across the oxygen minimum zone (OMZ) and with respect to oxygen fluctuations in the water column related to the El Niño Southern Oscillation (ENSO).
Sediments within the permanent OMZ are characterized by diffusive uptake and authigenic fixation of U, V and Mo as well as diffusive loss of Mn and Fe across the benthic boundary. Some of the dissolved Mn and Fe in the water column re-precipitate at the oxycline and shuttle particle-reactive trace metals to the sediment surface at the lower and upper boundary of the OMZ. At the lower boundary, pore waters are not sufficiently sulfidic as to enable an efficient authigenic V and Mo fixation. As a consequence, sediments below the OMZ are preferentially enriched in U which is delivered via both in situ precipitation and lateral supply of U-rich phosphorites from further upslope. Trace metal cycling on the Peruvian shelf is strongly affected by ENSO-related oxygen fluctuations in bottom water. During periods of shelf oxygenation, surface sediments receive particulate V and Mo with metal (oxyhydr)oxides that derive from both terrigenous sources and precipitation at the retreating oxycline. After the recurrence of anoxic conditions, metal (oxyhydr)oxides are reductively dissolved and the hereby liberated V and Mo are authigenically removed. This alternation between supply of particle-reactive trace metals during oxic periods and fixation during anoxic periods leads to a preferential accumulation of V and Mo compared to U on the Peruvian shelf. The decoupling of V, Mo and U accumulation is further accentuated by the varying susceptibility to re-oxidation of the different authigenic metal phases. While authigenic U and V are readily re-oxidized and recycled during periods of shelf oxygenation, the sequestration of Mo by authigenic pyrite is favored by the transient occurrence of oxidizing conditions.
Our findings reveal that redox-sensitive trace metals respond in specific manner to short-term oxygen fluctuations in the water column. The relative enrichment patterns identified might be useful for the reconstruction of past OMZ extension and large-scale redox oscillations in the geological record.
Changes in the concentration and isotopic composition of the major constituents in seawater reflect changes in their sources and sinks. Because many of the processes controlling these sources and ...sinks are tied to the cycling of carbon, such records can provide insights into what drives past changes in atmospheric carbon dioxide and climate. Here, we present a stable strontium (Sr) isotope record derived from pelagic marine barite. Our δ
Sr record exhibits a complex pattern, first declining between 35 and 15 million years ago (Ma), then increasing from 15 to 5 Ma, before declining again from ~5 Ma to the present. Numerical modeling reveals that the associated fluctuations in seawater Sr concentrations are about ±25% relative to present-day seawater. We interpret the δ
Sr data as reflecting changes in the mineralogy and burial location of biogenic carbonates.
The accumulation of methane hydrate in marine sediments is controlled by a number of physical and biogeochemical parameters including the thickness of the gas hydrate stability zone (GHSZ), the ...solubility of methane in pore fluids, the accumulation of particulate organic carbon at the seafloor, the kinetics of microbial organic matter degradation and methane generation in marine sediments, sediment compaction and the ascent of deep-seated pore fluids and methane gas into the GHSZ. Our present knowledge on these controlling factors is discussed and new estimates of global sediment and methane fluxes are provided applying a transport-reaction model at global scale. The modeling and the data evaluation yield improved and better constrained estimates of the global pore volume within the modern GHSZ ( ≥ 44 × 1015 m3), the Holocene POC accumulation rate at the seabed (~1.4 × 1014 g yr−1), the global rate of microbial methane production in the deep biosphere (4−25 × 1012 g C yr−1) and the inventory of methane hydrates in marine sediments ( ≥ 455 Gt of methane-bound carbon).
Dissolved silicon isotope compositions have been analysed for the first time in pore waters (δ30SiPW) of three short sediment cores from the Peruvian margin upwelling region with distinctly different ...biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments. The δ30SiPW varies between +1.1‰ and +1.9‰ with the highest values occurring in the uppermost part close to the sediment–water interface. These values are of the same order or higher than the δ30Si of the biogenic opal extracted from the same sediments (+0.3‰ to +1.2‰) and of the overlying bottom waters (+1.1‰ to +1.5‰). Together with dissolved silicic acid concentrations well below biogenic opal saturation, our collective observations are consistent with the formation of authigenic alumino-silicates from the dissolving biogenic opal. Using a numerical transport-reaction model we find that approximately 24% of the dissolving biogenic opal is re-precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56μmolSicm−2yr−1. The fractionation factor between the precipitates and the pore waters is estimated at −2.0‰. Dissolved and solid cation concentrations further indicate that off Peru, where biogenic opal concentrations in the sediments are high, the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino-silicate phases.
A numerical model was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate-bearing surface sediments from a cold vent site situated at Hydrate Ridge, an ...accretionary structure located in the Cascadia Margin subduction zone. Steady state simulations were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with bacterial mats and are affected by strong fluid flow from below. The model results underline the dominance of advective fluid flow that forces a large inflow of methane from below (869 μmol cm
−2 a
−1) inducing high oxidation rates in the surface layers. Anaerobic methane oxidation is the major process, proceeding at a depth-integrated rate of 870 μmol cm
−2 a
−1. A significant fraction (14%) of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite. The total rate of carbonate precipitation (120 μmol cm
−2 a
−1) allows for the build-up of a massive carbonate layer with a thickness of 1 m over a period of 20,000 years. Aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane-charge fluids is high enough (≥10 cm a
−1) to maintain super-saturation with respect to this highly soluble carbonate phase. It precipitates much faster within the studied surface sediments than previously observed in abiotic laboratory experiments, suggesting microbial catalysis. The investigated station is characterized by high carbon and oxygen turnover rates (≈1000 μmol cm
−2 a
−1) that are well beyond the rates observed at other continental slope sites not affected by fluid venting. This underlines the strong impact of fluid venting on the benthic system, even though the flow velocity of 10 cm a
−1 derived by the model is relative low compared to fluid flow rates found at other cold vent sites. Non-steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow. Moreover, they reveal that efficient methane oxidation in these sediments prevents methane outflow into the bottom water over a wide range of fluid flow velocities (<80 cm a
−1). Only at flow rates exceeding approximately 100 cm a
−1, does dissolved methane break through the sediment surface to induce large fluxes of up to 5000 μmol CH
4 cm
2 a
−1 into the overlying bottom water.
Methane seepage from the upper continental slopes of Western Svalbard has previously been attributed to gas hydrate dissociation induced by anthropogenic warming of ambient bottom waters. Here we ...show that sediment cores drilled off Prins Karls Foreland contain freshwater from dissociating hydrates. However, our modeling indicates that the observed pore water freshening began around 8 ka BP when the rate of isostatic uplift outpaced eustatic sea-level rise. The resultant local shallowing and lowering of hydrostatic pressure forced gas hydrate dissociation and dissolved chloride depletions consistent with our geochemical analysis. Hence, we propose that hydrate dissociation was triggered by postglacial isostatic rebound rather than anthropogenic warming. Furthermore, we show that methane fluxes from dissociating hydrates were considerably smaller than present methane seepage rates implying that gas hydrates were not a major source of methane to the oceans, but rather acted as a dynamic seal, regulating methane release from deep geological reservoirs.
Controversy over the oxygen isotope composition of seawater began in the 1950's, since which time there has been no agreement over whether the oxygen isotope composition of the oceans has changed ...over time. Resolving this uncertainty would allow the
δ
18O values of demonstrably well preserved marine authigenic precipitates to be used to reconstruct surface climate trends back to early Archean times and would help towards a more quantitative description of Earth's global water cycle on geological time scales.
Isotopic studies of marine carbonate and silica reveal a trend of increasing
δ
18O values with decreasing age since the Archean. This trend has been interpreted by some to reflect a progressive increase in seawater
δ
18O through time; however, it is generally accepted on the basis of ophiolite studies and theoretical considerations that seawater
δ
18O cannot change significantly because of the buffering effects of ocean crust alteration at mid-ocean ridges. As a result many alternative interpretations have been proposed, including meteoric alteration; warmer paleoclimates; higher seawater pH; salinity stratification and biased sampling.
Here we review these interpretations in the light of an updated compilation of marine carbonate
δ
18O from around the world, covering the Phanerozoic and Precambrian rock records. Recent models of the geological water cycle demonstrate how long-term trends in chemical weathering and hydrothermal circulation can indeed influence the O-isotope composition of the global ocean to the extent necessary to explain the carbonate
δ
18O trend, with residual variation attributed to climatic fluctuations and post-depositional alteration.
We present the further development of an existing model of the geological water cycle. In the model, seawater
δ
18O increased from about −
13.3‰ to −
0.3‰ over a period of 3.4 Ga, with average surface temperatures fluctuating between 10 °C to 33 °C, which is consistent with known biological constraints. Similar temperature variations are also obtained, although with higher starting seawater
δ
18O composition, when more conservative approaches are used that take into account the systematic effects of diagenetic alteration on mean calcite
δ
18O values. In contrast to much published opinion, the average
δ
18O value of ocean crust in the model remained relatively unchanged throughout all model runs. Invariable ophiolite
δ
18O values can, therefore, not be used as a definitive argument against changing seawater
δ
18O.
The most likely explanation for the long-term trend in seawater
δ
18O invokes two stepwise increases in the ratio of high- to low-temperature fluid/rock interactions. An initial increase may have occurred close to the Archean–Proterozoic boundary, but a possibly more significant increase took place near the Proterozoic–Phanerozoic boundary. Possible explanations for extremely low seawater
δ
18O during the Archean include higher continental weathering rates caused by a combination of higher atmospheric
pCO
2 (necessarily combined with high CO
2 outgassing rates), a greater abundance of relatively easily weathered volcanic rocks in greenstone belts and partial emergence of spreading ridges. The second increase may have been caused by the suppression of low-temperature overprinting of ocean crust alteration by the formation of a thick sediment cover on ridge flanks due to the emergence of shelly plankton at the beginning of the Phanerozoic. Postulated increases in spreading ridge depths since the Archean would also have enhanced the efficiency of vertical heat flux and changed the depth at which hydrothermal fluids boil, both of which would favour high- over low-temperature interactions with time.
Benthic fluxes of dissolved silicon (Si) from sediments into the water
column are driven by the dissolution of biogenic silica (bSiO2) and
terrigenous Si minerals and modulated by the precipitation ...of authigenic Si
phases. Each of these processes has a specific effect on the isotopic
composition of silicon dissolved in sediment pore fluids, such that the
determination of pore fluid δ30Si values can help to decipher
the complex Si cycle in surface sediments. In this study, the δ30Si signatures of pore fluids and bSiO2 in the Guaymas Basin
(Gulf of California) were analyzed, which is characterized by high
bSiO2 accumulation and hydrothermal activity. The δ30Si
signatures were investigated in the deep basin, in the vicinity of a
hydrothermal vent field, and at an anoxic site located within the pronounced
oxygen minimum zone (OMZ). The pore fluid δ30Sipf
signatures differ significantly depending on the ambient conditions. Within
the basin, δ30Sipf is essentially uniform, averaging
+1.2±0.1 ‰ (1 SD). Pore fluid δ30Sipf values from within the OMZ are significantly lower
(0.0±0.5 ‰, 1 SD), while pore fluids close to the
hydrothermal vent field are higher (+2.0±0.2 ‰,
1SD). Reactive transport modeling results show that the δ30Sipf is mainly controlled by silica dissolution (bSiO2 and
terrigenous phases) and Si precipitation (authigenic aluminosilicates).
Precipitation processes cause a shift to high pore fluid δ30Sipf signatures, most pronounced at the hydrothermal site.
Within the OMZ, however, additional dissolution of isotopically depleted Si
minerals (e.g., clays) facilitated by high mass accumulation rates of
terrigenous material (MARterr) is required to promote the low δ30Sipf signatures, while precipitation of authigenic
aluminosilicates seems to be hampered by high water ∕ rock ratios. Guaymas OMZ
δ30Sipf values are markedly different from those of the
Peruvian OMZ, the only other marine OMZ setting where Si isotopes have been
investigated to constrain early diagenetic processes. These differences
highlight the fact that δ30Sipf signals in OMZs worldwide
are not alike and each setting can result in a range of δ30Sipf values as a function of the environmental conditions. We
conclude that the benthic silicon cycle is more complex than previously
thought and that additional Si isotope studies are needed to decipher the
controls on Si turnover in marine sediment and the role of sediments in the
marine silicon cycle.
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