Kathleen C. Ruttenberg

AUTHIGENIC APATITE FORMATION AND BURIAL IN SEDIMENTS FROM NON-UPWELLING, CONTINENTAL MARGIN ENVIRONMENTS

Abstract Evidence for precipitation of authigenic carbonate fluorapatite (CFA) in Long Island Sound and Mississippi Delta sediments suggests that formation of CFA is not restricted to environments of active coastal upwelling. We present porewater data suggestive of CFA formation in both these areas. Application of a sequential leaching procedure, designed specifically to separate authigenic carbonate fluorapatite from other phosphorus-containing phases, including detrital apatite of igneous or metamorphic origin, provides strong supporting evidence for authigenic apatite formation in these sediments. The size of the authigenic apatite reservoir increases with depth, indicating continued formation of CFA during early diagenesis. This depth increase is mirrored by a decrease in solid-phase organic P at both sites, suggesting that CFA is forming at the expense of organic P. Mass balance considerations, application of diagenetic models to interstitial water nutrient data and the saturation state of the interstitial water are consistent with this interpretation. Diagenetic redistribution of phosphorus among the different solid-phase reservoirs is observed at both sites and results in near perfect retention of P by these sediments over the depth intervals sampled. Formation of CFA in continental margins which do not conform to the classically defined regions of phosphorite formation renders CFA a quantitatively more important sink than has previously been recognized. Including this reservoir as a newly identified sink for reactive P in the ocean, the residence time of P in the modern ocean must be revised downward. The implication for ancient oceans of CFA formation in continental margin sediments other than phosphorites is that phosphorite formation may be less a representation of episodicity in removal of reactive P from the oceans than of localized concentration of CFA in phosphatic sediments by secondary physical processes.

A reassessment of the sources and importance of land-derived organic matter in surface sediments from the Gulf of Mexico

Organic matter in surface sediments from two onshore-offshore transects in the northwestern Gulf of Mexico was characterized by a variety of techniques, including elemental, stable carbon, radiocarbon, and molecular-level analyses. In spite of the importance of the Mississippi River as a sediment source, there is little evidence for a significant terrigenous input based on the low carbon:nitrogen ratios (8 -5) and the enriched d 13 C values of bulk sedimentary organic carbon (219.7‰ to 221.7‰). Radiocarbon analyses, on the other hand, yield depleted D 14 C values (2277‰ to 2572‰) which indicate that a significant fraction of the sedimentary organic carbon (OC) in all these surface sediments must be relatively old and most likely of allochthonous origin. CuO oxidations yield relatively low quantities of lignin products (0.4 -1.4 mg/100 mg OC) along with compounds derived from proteins, polysaccharides, and lipids. Syringyl:vanillyl and cin- namyl:vanillyl ratios (averaging 1.6 and 0.5, respectively) and acid:aldehyde ratios for both vanillyl and syringyl phenols (averaging 0.8 and 1.2, respectively) indicate that the lignin present in sediments originates from nonwoody angiosperm sources and is highly degraded. The d 13 C values of lignin phenols in shelf sediments are relatively depleted in 13 C (averaging 226.3‰) but are increasingly enriched in 13 C at the slope sites (averaging 217.5‰ for the two deepest stations). We interpret these molecular and isotopic compositions to indicate that a significant fraction (

Sources and contribution of terrigenous organic carbon to surface sediments in the Gulf of Mexico

50%) of the lignin and, by inference, the land-derived organic carbon in northwestern Gulf of Mexico sediments ultimately originated from C4 plants. The source of this material is likely to be soil organic matter eroded from the extensive grasslands of the Mississippi River drainage basin. Notably, the mixed C4 and C3 source and the highly degraded state of this material hampers its recognition and quantification in shelf and slope sediments. Our data are consistent with higher than previously estimated inputs of land-derived organic carbon to regions of the ocean, such as the Gulf of Mexico, with significant sources of terrigenous C4-derived organic matter. Copyright

Importance of suspended participates in riverine delivery of bioavailable nitrogen to coastal zones

The sources and burial processes of organic matter in marine sediments are not well understood, yet they are important if we are to have a better understanding of the global carbon cycle. In particular, the nature and fraction of the terrestrial organic carbon preserved in marine sediments is poorly constrained. Here we use the chemical and stable carbon isotope signatures of oxidation products from a macromolecular component (lignin) of the terrigenous organic matter preserved in offshore surface sediments in the Gulf of Mexico to complement similar data from an existing onshore transect in this region. The complete onshore–offshore data set, along with radiocarbon dates of the bulk organic material at the same sites, allows the differentiation of material originating from plants that photosynthesize using the C4 mechanism from those that undergo C3 photosynthesis. We conclude that the offshore lignins derive from erosion of the extensive grassland (C4) soils of the Mississippi River drainage basin, and that the nearshore lignins originate largely from C3 plant detritus from coastal forests and swamps. This distribution is probably due to the hydrodynamic sorting of the different source materials during their seaward transport. These results suggest that previous studies have significantly underestimated the terrigenous fraction of organic matter in offshore sediments by not recognizing the contribution of C4 vegetation to the carbon-isotope composition. Such an underestimate may force revisions in the assessment of past marine primary productivity and associated organic carbon fluxes, and of organic matter preservation/remineralization and nutrient cycling in marine sediments.

Porewater pH and authigenic phases formed in the uppermost sediments of the Santa Barbara Basin

Total nitrogen (TN) loadings in riverine sediments and their coastal depocenters were compared for 11 river systems worldwide to assess the potential impact of riverine particulates on coastal nitrogen budgets. Strong relationships between sediment specific surface area and TN allow these impacts to be estimated without the intense sampling normally required to achieve such budgets. About half of the systems showed higher nitrogen loadings in the riverine sediments than those from the coastal depocenter. In spite of uncertainties, these comparisons indicate that large, turbid rivers, such as the Amazon, Huanghe, and the Mississippi, deliver sediments that in turn release significant or major fractions of the total riverine nitrogen delivery. Riverine particulates must therefore be considered an essential factor in watershed nutrient loading to coastal ecosystems and may affect delivered nutrient ratios as well as total nutrient loading. The relative importance of particulate versus dissolved delivery has decreased over recent decades in the Mississippi as a result of damming and fertilizer use in the watershed.

The Nature of Phosphorus Burial in Modern Marine Sediments

Abstract In this paper porewater and solid phase analyses are used in combination with in situ O 2 and pH microelectrode measurements to characterize early diagenetic processes in the uppermost sediments of the Santa Barbara Basin, California. Rapid reduction of dissolved oxygen, nitrate, solid phase manganese and iron, and dissolved sulfate is observed. Between sediment depths of 0 and 2 cm, reductive solubilization of ferric iron phases releases Fe 2+ , adsorbed phosphate, and fluoride to the porewaters and contributes to a sharp increase in porewater pH. Between 2 and 4 cm, sulfate reduction rates peak, pH levels off, and acid volatile sulfides and pyrite become the dominant forms of solid phase iron. Saturation state calculations, which depend largely on pH, indicate that the porewaters of the Santa Barbara Basin become saturated with respect to carbonate fluorapatite and calcite within the first 0.25 mm of the sediment and are highly supersaturated by and below 2 cm. In spite of this result, porewater evidence of phosphate and fluoride removal into a solid phase is observed only in the first ∼5 cm of some cores, whereas dissolved Ca profiles suggest dispersed calcite precipitation throughout the sediment column. This finding is interpreted as an indication of the nonsteady state nature of the surface reactions that may, given sufficient nucleation sites and time, lead to carbonate fluorapatite genesis in anoxic sediments. Finally, microelectrode pH profiles from two other basins in the California Borderlands are presented. These demonstrate that the porewaters of the Santa Barbara Basin are more alkaline than those of other basins. This outcome is attributed to the lack of particle mixing and a unique interplay between Fe liberation and FeS precipitation reactions in the Santa Barbara Basin.

Benthic recycling of biogenic debris in the eastern tropical Atlantic Ocean

Phosphorus is a key element in biogeochemical cycles because of its role as an essential nutrient. Because of the ability of certain marine organisms, such as cyanobacteria, to fix nitrogen, it has been normally assumed that the long term limiting factor in global oceanic productivity is phosphorus (e.g. Holland, 1978). Thus, a knowledge of phosphorus chemistry in the ocean is a key to a better understanding of the cycling of carbon, nitrogen, sulfur, and other bio-elements. Over shorter time scales, days to millenia, the cycle of phosphorus in the ocean is controlled by a combination of processes involving oceanic circulation, biosynthesis, sinking of particles, and bacterial regeneration of the particles at depth or on the sea floor (e.g. Berger et al., 1989). Consequently, concentrations of dissolved phosphorus in seawater vary from place to place as a result of the interaction of these processes. In this context sediments are important only as their surficial portions release additional dissolved phosphate to the overlying water. By contrast, sediments become much more important on longer time scales of tens of thousands to millions of years, because they are the ultimate repository for the removal of phosphate from the oceans. The overall level of phosphorus in the ocean, and therefore global biological productivity, is controlled by the long-term balance between input via rivers and output via burial in sediments.

Cross-flow filtration of dissolved and colloidal nitrogen and phosphorus in seawater: results from an intercomparison study

Abstract Distributions of pore water O2, NO−2, NO−3, NH+4, Si(OH)4, PO3−4, Mn2+, F−, and T.A. were determined at 15 stations in the eastern equatorial Atlantic. While overall profile characteristics are consistent with previous models of organic matter diagenesis, profile shapes suggest that a deep reaction layer, rich in organic C, is also present at many sites. While it is unlikely that the oxidation of organic C in this layer has had a major effect on the ocean C cycle, pore water profile shapes are significantly altered. Despite exposure to seawater SO2−4 concentrations for > 1000 years, decomposition of the organic matter in the layer appears to be restricted to oxic and suboxic processes. These results suggest major differences in organic carbon decomposition and preservation under oxic/suboxic and anoxic conditions. Present-day benthic fluxes are largest adjacent to the eastern boundary coastal upwelling region and similar in magnitude to values reported for the eastern Pacific. Preliminary estimates suggest that the benthic respiration in the eastern 1 3 of the North Atlantic south of 20°N may alone account for >20% of the total deep North Atlantic respiration. Combining these results with estimates of organic C burial and deep water-column decomposition suggests that this region is a major location of organic C input into the deep sea.

Dissolved Organic Phosphorus Production during Simulated Phytoplankton Blooms in a Coastal Upwelling System.

A cross-flow filtration (CFF) intercomparison study was conducted to evaluate the effectiveness of a variety of CFF systems in fractionating and recovering ≥ 1-kD (kilodalton) molecular weight seawater organic matter. Inorganic nutrients and total and organic nitrogen and phosphorus results are presented for CFF-processed, 0.2-μm-filtered seawater representative of both shallow coastal (Woods Hole) and deep open-ocean (Hawaii) environments. Concentrations of NO3− + NO2−, NH4+, PO43− and Si(OH)4 all showed evidence of contamination or scavenging within individual CFF systems at one time or another. When adequate precautions were taken, however, nutrients displayed predicted conservative behavior in all systems. Organic N was generally observed to be less of a potential contaminant in CFF systems than was organic C. Contamination by inorganic and organic P was relatively common. Due to the low natural abundances of P in these and other natural seawater samples, even slight P contamination may confound interpretation of colloidal P recoveries. Those CFF systems that recovered significant quantities of colloidal (≥ 1-kD molecular weight) organic N generally recovered signficant quantities of colloidal organic C as well. Recoveries of colloidal organic N ranged from 0 to ~ 50% of total seawater organic N. High variability in colloidal organic N recovery (~ ± 20%) was characteristic of identical or nearly identical CFF systems. High variability was also observed in the recovery of colloidal organic P (range = 20–80% of total seawater organic P) from coastal Woods Hole seawater. Using open-ocean Hawaii seawater, in which no total seawater organic P was detected, CFF systems that recovered significant quantities of colloidal organic C and N showed recovery of an apparent colloidal organic P fraction. This finding suggests that scavenging of P into the colloidal fraction may occur during CFF processing. CN elemental ratios of CFF-processed organic fractions showed several differences between Woods Hole and Hawaii seawater. In general, CN values for coastal Woods Hole seawater organic matter were relatively similar in the various fractions compared to the unfractionated, starting seawater. In contrast, the high-molecular-weight (≥ 1 kD) fraction of open-ocean Hawaii seawater had highly elevated CN values relative to both unfractionated seawater and low-molecular-weight (≤ 1 kD) organic matter. It was not possible to adequately assess CP ratios at this time due to potential artifact-associated ambiguities in organic P data. These results emphasize the need for greater controls and calibration both among and within different makes of CFF systems. Excessive variability in both quantitative recoveries and qualitative characterization of organic matter isolated by CFF. It is therefore not possible at present to exclude the possibility of fractionation artifacts during the CFF processing of seawater organic matter. Additional work is needed to validate the use of CFF as a tool for elucidating the characteristics of seawater organic matter and the importance of different dissolved and colloidal fractions to the seawater microbial community.

Large-scale climatic effects on traditional Hawaiian fishpond aquaculture

Dissolved organic phosphorus (DOP) is increasingly recognized as an important phosphorus source to marine primary producers. Despite its importance, the production rate and fate of DOP is poorly understood. In this study, patterns of DOP production were evaluated by tracking the evolution of DOP during simulated phytoplankton blooms initiated with nutrient amended surface waters, relative to controls, from the Oregon (USA) coastal upwelling system. Nitrogen (N) and phosphorus (P) additions were used to decouple DOP production and hydrolysis by inducing or repressing, respectively, community alkaline phosphatase activity. In order to examine the progression of nutrient uptake and DOP production under upwelling versus relaxation conditions, two experiments were initiated with waters collected during upwelling events, and two with waters collected during relaxation events. Maximum [under (+P) conditions] and minimum [under (+N) conditions] DOP production rates were calculated and applied to in situ DOP levels to evaluate which end-member rate most closely approximates the in situ DOP production rate at the four study sites in this coastal system. Increases in DOP concentration occurred by day-5 in control treatments in all experiments. N treatments displayed increased chlorophyll a, increased alkaline phosphatase activity, and yielded lower net DOP production rates relative to controls, suggesting that DOP levels were depressed as a consequence of increased hydrolysis of bioavailable DOP substrates. Phosphorus additions resulted in a significant net production of DOP at all stations, but no increase in chlorophyll a relative to control treatments. The contrasting patterns in DOP production between treatments suggests that changes in the ambient dissolved inorganic nitrogen:dissolved inorganic phosphorus (DIN:DIP) ratio could exert profound control over DOP production rates in this system. Patterns of DOP production across the different experiments also suggest that bathymetry-driven differences in water residence times can influence DOP cycling. Taken together, these factors may impact the potential export of DOP to offshore ecosystems.