Santa Monica Basin Research Articles

Diurnal Variability of Ocean Temperature Off Pt. Sal, Central California

AbstractDiurnal‐band temperature (DU‐T) variability is investigated over 30 days from measures of temperature over the vertical obtained by 78 moorings deployed in the Santa Monica Basin near Pt. Sal (PS), California. DU‐T variability accounts for 3.6%–23% of the total variability. Vertical empirical orthogonal function mode‐1 DU‐T variability accounts for an average of 86% of the total vertical DU‐T variability at each mooring, is greatest north of PS where its vertical structure is baroclinic, and weakest south of PS where its vertical structure is surface intensified. Spatial complex empirical orthogonal function mode‐1 DU‐T accounts for 73% of the total DU‐T over the region, reveals an onshore baroclinic DU‐T propagation of ∼0.5 m/s, and a previously observed equatorward baroclinic DU‐T propagation of ∼1.9 m/s along the coast north of PS. Dominant vertical mode‐1 diurnal‐band baroclinic velocity (DU‐w) variability at each mooring exhibits a two‐layer structure that is 180° out of phase with maxima near the surface and the bottom, and resembles the DU‐T propagation. DU‐T variability mechanisms include wind‐forced deepening of the surface mixed layer, inertial motions, upwelling, and barotropic and baroclinic tides. Among these concomitant diurnal mechanisms, wind‐driven inertial motions are likely the primary mechanism influencing DU‐T variability during upwelling‐favorable conditions owing to coherence between observed DU‐w and the predicted inertial response to wind‐forcing. Further, spatial mode‐1 DU‐T is correlated and coherent with spatial mode‐1 DU‐w and diurnal wind‐forcing. In addition to describing predominant DU‐T variability patterns, these findings have important implications regarding spatial patterns in biological recruitment, transport, and settlement.

Early diagenetic processes in an iron-dominated marine depositional system

The early diagenetic interplay between reactive iron, sulfur, and organic matter in the bathymetrically isolated Santa Monica Basin (SMB) sediments are investigated in this study. We explore solid-phase and porewater profiles from the basin, supplemented with a transect from 71 to 907 m water depth that includes oxygenated (>60 μM O2) bottom waters near the coast and oxygen-deficient waters (∼4 µM O2) in the basin. The geochemical data of the basin sediments are further scrutinized by means of reactive transport modeling.The results show that the basin sediments do not follow the traditional geochemical signatures of oxygen-deficient settings. A lack of dissolved sulfide accumulation and sulfurized iron persists despite the sediments being deposited under reducing conditions (without bioturbation/bioirrigation), strong organic carbon input (TOC up to 5.0 wt%), and active dissimilatory sulfate reduction. Not only did we find an exceptional enrichment in highly reactive Fe in the surface sediments (∼45 % of total Fe), but the enrichment of reactive Fe, including ferrihydrite, persists downcore and coexists with high levels of dissolved Fe. The enhanced preservation of Fe oxides and lack of iron-sulfide precipitation is in part explained by detection via Mössbauer spectra of iron oxides bounded to organic matter (Fe[III]-OM coprecipitates).The modeled Fe budget shows that most of the Fe oxides in the surface sediments are internally recycled by upward diffusion and subsequent oxidation of Fe2+. Sulfide oxidation coupled to Fe reduction effectively precludes sulfide accumulation while enhancing build-up of dissolved Fe, fueling the Fe cycle within the first 5 cm depth. Continuous reoxidation of Fe2+ enhances the formation of Fe(III)-OM coprecipitates, limiting the amount of reactive organic matter. In the unavailability of labile organic matter, other than within the uppermost layers, the organic-rich sediment profiles are dominated by Fe cycling that limits the production and preservation of sulfides and enhances the preservation of Fe oxides and organic carbon. This study highlights key local controls on Fe availability in marginal basins and describes an intricate biogeochemical C-Fe-S cycling in modern and possibly ancient marine systems with important implications for Fe availability in the marine realm.

The Paleogene Gosau Group Slope Basins of the Incipient Eastern Alpine Orogenic Wedge: A Case Study at the Gams Basin (Austria)

This study investigates the Paleogene deep-water depositional system of the Gosau Group at Gams, Styria (Austria). The examined sections of the Danian to the Ypresian age (NP1–NP12) comprise sediments of the Nierental and Zwieselalm Formations. Four deep-water clastic facies assemblages were encountered: (1) pelagic marls with thin turbidites, (2) carbonate-rich turbidites, (3) carbonate-poor turbidites, and (4) marl-bearing turbidites; slump beds and mass flow deposits are common features in all facies assemblages. Based on heavy mineral, thin section, microprobe, and paleoflow analyses, provenance was from the surrounding Northern Calcareous Alps (NCA) rocks and exhuming metamorphic Upper Austroalpine units to the south. In addition, biogenic calcareous material was delivered by adjacent contemporaneous shelf zones. The sedimentary depocenter was situated at the slope of the incipient Alpine orogenic wedge, in frontal parts of the NCA, facing the subducting Penninic Ocean/Alpine Tethys. The evolution of the Gams Basin was connected to the eoalpine and mesoalpine orogeny and the adjunctive transpressional setting. The Gams deep-water depositional system is interpreted as an aggrading or prograding submarine fan, deposited into a small confined slope basin, positioned along an active continental margin, bound and influenced by (strike-slip) faults, related to crustal shortening. The development of the Gams slope basin and its infilling sequences was mainly dominated by tectonism and sediment supply, rather than by eustatic sea-level fluctuations. The basin was cut off during the Eocene due to renewed orogeny. A Quaternary analogue for the Upper Cretaceous to Paleogene basin setting of the Gams area is represented by the Santa Monica Basin in the California Continental Borderland.

Evidence of changes in sedimentation rate and sediment fabric in a low-oxygen setting: Santa Monica Basin, CA

Abstract. The Southern California Bight is adjacent to one of the world's largest urban areas, Los Angeles. As a consequence, anthropogenic impacts could disrupt local marine ecosystems due to municipal and industrial waste discharge, pollution, flood control measures, and global warming. Santa Monica Basin (SMB), due to its unique setting in a low-oxygen and high-sedimentation environment, can provide an excellent sedimentary paleorecord of these anthropogenic changes. This study examined 10 sediment cores, collected from different parts of the SMB between spring and summer 2016, and compared them to existing cores in order to document changes in sedimentary dynamics during the last 250 years, with an emphasis on the last 40 years. The 210Pb-based mass accumulation rates (MARs) for the deepest and lowest oxygen-containing parts of the SMB basin (900–910 m) have been remarkably consistent during the past century, averaging 17.1±0.6 mg cm−2 yr−1. At slightly shallower sites (870–900 m), accumulation rates showed more variation but yield the same accumulation rate, 17.9±1.9 mg cm−2 yr−1. Excess 210Pb sedimentation rates were consistent with rates established using bomb test 137Cs profiles. We also examined 14C profiles from two cores collected in the deepest part of the SMB, where fine laminations are present up to about 450 yr BP. These data indicate that the MAR was slower prior to ∼1900 CE (rates obtained were 9 and 12 mg cm−2 yr−1). The δ13Corg profiles show a relatively constant value where laminations are present, suggesting that the change in sediment accumulation rate is not accompanied by a change in organic carbon sources to the basin. The increase in sedimentation rate towards the Recent occurs at about the time previous studies predicted an increase in siltation and the demise of a shelly shelf benthic fauna on the SMB shelf. X-radiographs show finely laminated sediments in the deepest part of the basin only, with centimeter-scale layering of sediments or no layering whatsoever in shallower parts of the SMB basin. The absence of finely laminated sediments in cores MUC 10 (893 m) and MUC 3 (777 m) suggests that the rate at which anoxia is spreading has not increased appreciably since cores were last analyzed in the 1980s. Based on core top data collected during the past half century, sedimentary dynamics within SMB have changed minimally during the last 40 years. Specifically, mass accumulation rates, laminated sediment fabric, extent of bioturbation and % Corg have not changed. The only parameter that appeared to have changed in the last 450 years was the MAR, with an apparent > 50 % increase occurring between ∼1850 CE and the early 1900s. The post-1900 CE constancy of sedimentation through a period of massive urbanization in Los Angeles is surprising.

Iron redox cycling, sediment resuspension and the role of sediments in low oxygen environments as sources of iron to the water column

Continental margin sediments represent a potentially large, but poorly constrained, source of iron to surface waters to support primary productivity. To investigate this problem, we examined iron redox cycling in the sediments of Santa Monica Basin (SMB), CA as it relates to benthic sources of iron to the water column. Our results show that iron redox cycling in SMB sediments results in the formation of a thin layer at the sediment surface (<2 cm thick) that is highly enriched in amorphous, reactive iron oxides. Such oxides, when resuspended into the water column, may be a source of bioaccessible iron to primary producers. Calculations show that the resuspension flux of reactive amorphous iron oxides from SMB sediments may be up to two orders of magnitude greater than benthic fluxes of soluble dissolved iron from most continental margin sediments. The factors which favor the formation of this surface sediment layer enriched in reactive amorphous iron oxides appear to be related to the dissolved O2 level in SMB bottom waters, and its impact on iron redox cycling and the occurrence of sediment bioturbation. These factors are not necessarily unique to SMB, and other sediment sites on the open continental margin that lie within the O2 minimum zone may also have similar properties. Furthermore, expansion of O2 minimum zones due to global warming could possibly increase the areal extent of such environments, and potentially act as a negative feedback on rising atmospheric CO2 by providing additional iron to stimulate primary production. Further work examining iron biogeochemistry in low O2 continental margin settings will be needed to more critically examine these possibilities, along with studies that better define the role sediment resuspension fluxes may play as an iron source to surface waters. Such efforts will ultimately lead to an improved understanding of the role that ocean deoxygenation may play in enhancing this sediment iron source to support surface ocean primary production.

Carbon isotopic heterogeneity of coenzyme F430 and membrane lipids in methane‐oxidizing archaea

Archaeal ANaerobic MEthanotrophs (ANME) facilitate the anaerobic oxidation of methane (AOM), a process that is believed to proceed via the reversal of the methanogenesis pathway. Carbon isotopic composition studies indicate that ANME are metabolically diverse and able to assimilate metabolites including methane, methanol, acetate, and dissolved inorganic carbon (DIC). Our data support the interpretation that ANME in marine sediments at methane seeps assimilate both methane and DIC, and the carbon isotopic compositions of the tetrapyrrole coenzyme F430 and the membrane lipids archaeol and hydroxy-archaeol reflect their relative proportions of carbon from these substrates. Methane is assimilated via the methyl group of CH3 -tetrahydromethanopterin (H4 MPT) and DIC from carboxylation reactions that incorporate free intracellular DIC. F430 was enriched in 13 C (mean δ13 C=-27‰ for Hydrate Ridge and -80‰ for the Santa Monica Basin) compared to the archaeal lipids (mean δ13 C=-97‰ for Hydrate Ridge and -122‰ for the Santa Monica Basin). We propose that depending on the side of the tricarboxylic acid (TCA) cycle used to synthesize F430, its carbon was derived from 76% DIC and 24% methane via the reductive side or 57% DIC and 43% methane via the oxidative side. ANME lipids are predicted to contain 42% DIC and 58% methane, reflecting the amount of each assimilated into acetyl-CoA. With isotope models that include variable fractionation during biosynthesis for different carbon substrates, we show the estimated amounts of DIC and methane can result in carbon isotopic compositions of-73‰ to-77‰ for F430 and-105‰ for archaeal lipids, values close to those for Santa Monica Basin. The F430 δ13 C value for Hydrate Ridge was 13 C-enriched compared with the modeled value, suggesting there is divergence from the predicted two carbon source models.

Discovery of an Extensive Deep-Sea Fossil Serpulid Reef Associated With a Cold Seep, Santa Monica Basin, California

Multibeam bathymetric mapping of the Santa Monica Basin in the eastern Pacific has revealed the existence of a number of elevated bathymetric features, or mounds, harboring cold seep communities. During 2013-2014, mounds at ~600 m water depth were observed for the first time and sampled by Monterey Bay Aquarium Research Institute’s ROV Doc Ricketts. Active cold seeps were found, but surprisingly one of these mounds was characterized by massive deposits composed of fossil serpulid worm tubes (Annelida: Serpulidae) exhibiting various states of mineralization by authigenic carbonate. No living serpulids with equivalent tube morphologies were found at the site; hence the mound was termed ‘Fossil Hill’. In the present study, the identity of the fossil serpulids and associated fossil community, the ages of fossils and authigenic carbonates, the formation of the fossil serpulid aggregation, and the geological structure of the mound are explored. Results indicate that the tubes were most likely made by a deep-sea serpulid lineage, with radiocarbon dating suggesting that they have a very recent origin during the Late Pleistocene, specifically to the Last Glacial Maximum ~20,000 years ago. Additional U-Th analyses of authigenic carbonates mostly corroborate the radiocarbon dates, and also indicate that seepage was occurring while the tubes were being formed. We also document similar, older deposits along the approximate trajectory of the San Pedro Basin Fault. We suggest that the serpulid tube facies formed in situ, and that the vast aggregation of these tubes at Fossil Hill is likely due to a combination of optimal physical environmental conditions and chemosynthetic production, which may have been particularly intense as a result of sea-level lowstand during the Last Glacial Maximum.

Bottom-water oxygenation and environmental change in Santa Monica Basin, Southern California during the last 23 kyr

The Southern California Borderland is a region that experiences strong natural variations in bottom water oxygen and pH. We use marine sediments from Santa Monica Basin to reconstruct environmental conditions in the basin's bottom water from the Last Glacial Maximum (LGM) to present. We then compare the records to the adjacent Santa Barbara Basin and Santa Lucia Slope. High-resolution records of benthic foraminiferal oxygen and carbon isotopes (δ18O and δ13C), benthic foraminiferal assemblages, and bulk sedimentary organic matter geochemistry records exhibit major changes associated with late Quaternary millennial-scale global climate oscillations. Our data show the dominance of low‑oxygen benthic foraminiferal assemblages during warm intervals, and assemblages representing higher dissolved oxygen during cooler intervals, as also seen in Santa Barbara Basin and Santa Lucia Slope. However, our record shows a stronger and longer-lasting oxygen minimum zone (OMZ) between the mid Younger Dryas (YD) and the Early Holocene than at neighboring sites, indicated by dominance of Bolivina tumida (characteristic of major hypoxia) in the assemblage. The middle to late Holocene (from ~9 to 0kyr) had weaker hypoxia than the early Holocene, with assemblages mainly composed of Bolivina argentea and Uvigerina peregrina. Santa Monica Basin remained slightly hypoxic throughout the past 23kyr, however, differences in the degree of hypoxia from Santa Barbara Basin and Santa Lucia Slope (especially from the B-A to the Early Holocene) are seen. The Santa Monica Basin bottom water is affected by regional processes, such as changes in the source of intermediate water and/or changing ventilation (oxygenation) of the intermediate water source. This is due to the greater depth and the more southern geographic position of the Basin, which reduces exposure to the oxygenated North Pacific Intermediate Water current. Additional local processes also affect the basin, such as the effects of sediment influx from submarine canyons. This analysis utilizing parallel geochemical and micropaleontological records brings new insights into bottom water and climate conditions in Santa Monica Basin, indicating regional similarities and differences from adjacent basins, and provides insight into the causes for changes in bottom water oxygenation.

Ecophenotypic responses of benthic foraminifera to oxygen availability along an oxygen gradient in the California Borderland

AbstractSpatial variation in environmental conditions can elicit predictable size and morphological responses in marine organisms through influences on physiology. Thus, spatial and temporal variation in marine organism size and shape are often used to infer paleoenvironmental conditions, such as dissolved oxygen concentrations. Benthic foraminifera commonly serve as a tool for reconstructing past ocean oxygen levels. For example, benthic foraminiferal species assemblages, within‐ and among‐species patterns of test morphology, and geochemical analyses of carbonate tests are often employed to reconstruct past marine conditions. In this study, we measured the sizes and shapes of modern foraminifera representing four species that inhabit a steep oxygen gradient in the Santa Monica Basin on the Southern California Borderland with the aim of quantifying the influence of oxygen availability on foraminiferal morphology, both within and among species. Most foraminifera rely on aerobic respiration, but the four benthic foraminifera from the Santa Monica Basin do not show the predicted size and morphological responses to variations in dissolved oxygen concentrations based on first principles of cell physiology: Bolivina spissa shows no volume or volume‐to‐surface area response, Uvigerina peregrina increases in both test volume and volume‐to‐surface area ratio with decreasing dissolved oxygen concentrations, and both Bolivina argentea and Loxostomum pseudobeyrichi decrease in test volume, but only L. pseudobeyrichi shows a decrease in test volume‐to‐surface area ratio with decreasing oxygen concentrations as expected from physiological predictions. These findings imply that the morphological responses of individual foraminiferal species are not necessarily representative of the responses of other foraminiferal species within the community. Our findings further suggest that these species use physiological strategies such as depressed metabolic rates and alternative energy metabolisms to persist in low oxygen environments and, therefore, cannot be used in any simple way as paleo‐oxygen indicators. Should Proterozoic (1,000–542 million years ago) protists have possessed metabolic strategies similar to foraminifera, the sizes and shapes of protists in the fossil record may not usefully constrain ambient oxygen conditions during the appearance and initial taxonomic radiation of heterotrophic eukaryotes.

Recycling of Organic Matter in the Sediments of Santa Monica Basin, California Borderland

Geochemical and isotopic data for the uppermost 1.2 m of the sediments of the central Santa Monica Basin plain were examined to better understand organic matter deposition and recycling at this site. Isotopic signatures (Δ14C and δ13C) of methane (CH4) and dissolved inorganic carbon (DIC) indicate the occurrence of anaerobic oxidation of CH4 that is fueled by CH4 supplied from a relict reservoir that is decoupled from local organic carbon (Corg) degradation and methanogenesis. This finding was corroborated by a flux budget of pore-water solutes across the basal horizon of the profile. Together these results provide a plausible explanation for the anomalously low ratio between alkalinity production and sulfate consumption reported for these sediments over two decades ago. Shifts in Δ14C and δ13C signatures of Corg have previously been reported across the 20-cm depth horizon for this site and attributed to a transition from oxic to anoxic bottom water that occurred ~350 years BP. However, we show that this horizon also coincides with a boundary between the base of a hemipelagic mud section and the top of a turbidite interval, complicating the interpretation of organic geochemical data across this boundary. Radiocarbon signatures of DIC diffusing upward into surface sediments indicate that remineralization at depth is supported by relatively 14C-enriched Corg within the sedimentary matrix. While the exact nature of this Corg is unclear, possible sources are hemipelagic mud sections that were buried rapidly under thick turbidites, and 14C-rich moieties dispersed within Corg-poor turbidite sections.

In situ Raman measurement of HS− and H2S in sediment pore waters and use of the HS−:H2S ratio as an indicator of pore water pH

The smell of hydrogen sulfide upon recovery of deep sea cores from areas of reducing sediments is familiar to most ocean scientists and is simple testimony to the problem of dissolved gas loss during core recovery and processing. For this reason methods of in situ measurement of sulfide and other gases are keenly sought, and both microelectrode and spectroscopic measurements have been widely used. We report here on the use of a robust 50cm long titanium pore water probe combined with a laser Raman sensing system for in situ measurement of the sulfide concentration and sulfide speciation status of deep sea sediment pore waters. Uniquely the Raman signal spectroscopically senses and simultaneously resolves both the H2S (υ1 2592 Δcm−1) and HS− (υ1 2573 Δcm−1) species. We have calibrated the Raman response factors as a function of pH and found that the Raman cross section for HS− is a factor of 1.563 greater than for the H2S form. We have carried out stepwise profiling of the pore water chemistry (CH4, SO42−, H2S, HS−) of highly reducing sediments in the Santa Monica Basin and from the combined sulfide species we can assess total dissolved sulfide. Since the ratio of HS−:H2S is a well-defined function of pH with the pK for this equilibrium being close to 7.0, and since the pH of many sediment pore waters is also close to 7 then the observed HS−:H2S ratio provides an elegant pH sensitive “dye”. We have solved the required Raman-equilibrium relationships and show that the in situ pH of sulfide rich pore waters can be determined rapidly and directly from these observations. Several field examples are provided.

Vitamin B1 in marine sediments: pore water concentration gradient drives benthic flux with potential biological implications.

Vitamin B1, or thiamin, can limit primary productivity in marine environments, however the major marine environmental sources of this essential coenzyme remain largely unknown. Vitamin B1 can only be produced by organisms that possess its complete synthesis pathway, while other organisms meet their cellular B1 quota by scavenging the coenzyme from exogenous sources. Due to high bacterial cell density and diversity, marine sediments could represent some of the highest concentrations of putative B1 producers, yet these environments have received little attention as a possible source of B1 to the overlying water column. Here we report the first dissolved pore water profiles of B1 measured in cores collected in two consecutive years from Santa Monica Basin, CA. Vitamin B1 concentrations were fairly consistent between the two years ranging from 30 pM up to 770 pM. A consistent maximum at ~5 cm sediment depth covaried with dissolved concentrations of iron. Pore water concentrations were higher than water column levels and represented some of the highest known environmental concentrations of B1 measured to date, (over two times higher than maximum water column concentrations) suggesting increased rates of cellular production and release within the sediments. A one dimensional diffusion-transport model applied to the B1 profile was used to estimate a diffusive benthic flux of ~0.7 nmol m−2 d−1. This is an estimated flux across the sediment-water interface in a deep sea basin; if similar magnitude B-vitamin fluxes occur in shallow coastal waters, benthic input could prove to be a significant B1-source to the water column and may play an important role in supplying this organic growth factor to auxotrophic primary producers.

Distinguishing and understanding thermogenic and biogenic sources of methane using multiply substituted isotopologues

Sources of methane to sedimentary environments are commonly identified and quantified using the stable isotopic compositions of methane. The methane “clumped-isotope geothermometer”, based on the measurement of multiply substituted methane isotopologues (13CH3D and 12CH2D2), shows promise in adding new constraints to the sources and formational environments of both biogenic and thermogenic methane. However, questions remain about how this geothermometer behaves in systems with mixtures of biogenic and thermogenic gases and different biogenic environments. We have applied the methane clumped-isotope thermometer to a mixed biogenic–thermogenic system (Antrim Shale, USA) and to biogenic gas from gas seeps (Santa Barbara and Santa Monica Basin, USA), a pond on the Caltech campus, and methanogens grown in pure culture. We demonstrate that clumped-isotope based temperatures add new quantitative constraints to the relative amounts of biogenic vs. thermogenic gases in the Antrim Shale indicating a larger proportion (∼50%) of thermogenic gas in the system than previously thought. Additionally, we find that the clumped-isotope temperature of biogenic methane appears related to the environmental settings in which the gas forms. In systems where methane generation rates appear to be slow (e.g., the Antrim Shale and gas seeps), microbial methane forms in or near both internal isotopic equilibrium and hydrogen-isotope equilibrium with environmental waters. In systems where methane forms rapidly, microbial methane is neither in internal isotopic equilibrium nor hydrogen-isotope equilibrium with environmental waters. A quantitative model of microbial methanogenesis that incorporates isotopes is proposed to explain these results.

Particulate δ15N in laminated marine sediments as a proxy for mixing between the California Undercurrent and the California Current: A proof of concept

AbstractThe measurements of particulate δ15N in coastal marine laminated sediments provide a high‐resolution proxy for fluctuations in the intensity of denitrification in the water column. In the eastern tropical North Pacific oxygen minimum zone, this denitrification signal is transported northward by the California Undercurrent, thus serving as a tracer of ocean circulation. This is verified through comparisons between salinity in the thermocline off Southern California (Santa Monica Basin) and the difference between δ15Nsed within age equivalent sediments from a southern (Pescadero Slope) and northern (Santa Monica Basin) site. Trends in this parameter, Δδ15Nsed, relate to Pacific Decadal Oscillation (PDO) phase changes between 1900 and 1990. We hypothesize that the decline in Δδ15Nsed during warm PDO phases is due to a strengthening of the California Undercurrent transporting 15N‐enriched nitrate from the eastern tropical North Pacific northward. The deviation from this trend after 1990 suggests recent changes in circulation and/or California Current water nutrient biogeochemistry.

High-Resolution Topography-Following Chemical Mapping of Ocean Hypoxia by Use of an Autonomous Underwater Vehicle: The Santa Monica Basin Example

AbstractThis paper reports on the execution of a combined chemical sensing/high-resolution terrain-following autonomous underwater vehicle (AUV) survey to explore the fine structure and functional boundaries of the Santa Monica Basin suboxic zone and its relationship to topography. An AUV mapping vehicle is used in a novel configuration—combining the mapping vehicle tail section, with precision inertial navigation and acoustic communications systems, with CTD/O2, NO3 sensing, and Gulper water sampling systems. The challenge was to perform a long-distance near-bottom physical/chemical survey in deep water without any intermediate surfacing to disrupt the survey or require the vehicle to surface in areas of heavy ship traffic. Some 210 km of AUV cruise track at ≈10 m above bottom were accomplished during a 3-day survey. The dissolved oxygen concentration [O2] data are combined with temperature T, salinity S, and hydrostatic pressure P to produce maps of oxygen partial pressure pO2 that help define the limits at which the oceanic supply of O2 can match the O2 demands required to sustain various forms of marine life. The chemical NO3 sensing was included to define the critical pO2 boundary at which NO3 reduction occurs. The combination of a high-resolution terrain-following AUV with chemical sensors is important for a diverse array of investigations, including the study of vent sites, and for locating the source of chemical signals originating from the seafloor. The hypoxic basin example here permits better discrimination between general climate/circulation controls on hypoxia and more specific point-source-driven processes.

Autonomous Application of Quantitative PCR in the Deep Sea: In Situ Surveys of Aerobic Methanotrophs Using the Deep-Sea Environmental Sample Processor

Recent advances in ocean observing systems and genomic technologies have led to the development of the deep-sea environmental sample processor (D-ESP). The D-ESP filters particulates from seawater at depths up to 4000 m and applies a variety of molecular assays to the particulates, including quantitative PCR (qPCR), to identify particular organisms and genes in situ. Preserved samples enable laboratory-based validation of in situ results and expanded studies of genomic diversity and gene expression. Tests of the D-ESP at a methane-rich mound in the Santa Monica Basin centered on detection of 16S rRNA and particulate methane monooxygenase (pmoA) genes for two putative aerobic methanotrophs. Comparison of in situ qPCR results with laboratory-based assays of preserved samples demonstrates the D-ESP generated high-quality qPCR data while operating autonomously on the seafloor. Levels of 16S rRNA and pmoA cDNA detected in preserved samples are consistent with an active community of aerobic methanotrophs near the methane-rich mound. These findings are substantiated at low methane sites off Point Conception and in Monterey Bay where target genes are at or below detection limits. Successful deployment of the D-ESP is a major step toward developing autonomous systems to facilitate a wide range of marine microbiological investigations.

A comparison of megafaunal communities in five submarine canyons off Southern California, USA

Remotely operated vehicle surveys were conducted in five submarine canyons off Southern California during research expeditions in 2005 and 2010. Video transects from a range of depths were analysed to produce presence/absence data of megafauna for each site. A comparison of benthic communities at various depths, locales, and canyons was performed. No significant difference was found between canyon communities based on the level of sediment transport activity, however this may be due to the unbalanced sampling of this opportunistic study. There was significant variation in biological community composition and abundance amongst water depths. These depth-related trends are in agreement with the findings of the previous studies and are likely tied to depth-correlated variables such as hydrostatic pressure, temperature, salinity, and oxygen concentration. Species richness was found to initially increase with depth before declining rapidly at the mouths of the studied canyons. Low oxygen levels in the Santa Monica Basin, into which four of the surveyed canyons empty, may explain this.

Dissolved organic carbon dynamics in anaerobic sediments of the Santa Monica Basin

Cycling of dissolved organic carbon (DOC) was investigated in anoxic sediments of the Santa Monica Basin, California Borderland, by analyzing the concentration and isotopic signatures (Δ14C and δ13C) of pore-water DOC and dissolved inorganic carbon (DIC), and organic compound classes extracted from the bulk sediments. DOC and DIC increased across the sediment–water interface, indicating net efflux of these solutes out of the sediments. Throughout the depth interval examined (0–30cm), the Δ14C value of DOC (Δ14CDOC) was similar to, or higher than, that of bulk sedimentary particulate organic carbon (POC), indicating degradation of relatively 14C-rich components of POC. There were prominent peaks in both Δ14CDOC and Δ14CDIC in the uppermost 2 cm of the sediment column, indicating degradation and remineralization of 14C-rich, labile organic matter in the near-surface sediments. However, below these sub-surface maxima, Δ14CDOC and Δ14CDIC decreased with depth by ∼200‰ and ∼50‰, respectively. Given the diffusive time scales, these decreases were too large to be explained by 14C loss due to radioactive decay. To help explain these observations, we constructed and implemented a selective degradation model that considers bulk pore-water DOC to be the sum of three kinetically- and isotopically-distinct sub-components. Based on this model, the most reactive DOC fraction, which supported ∼60% of the DIC production, had a Δ14C value indicating the presence of bomb-14C. The intermediate fraction had a Δ14C value of ∼−60‰ and accounted for most of the pore-water DOC standing stock. The least reactive fraction was virtually non-reactive in these sediments, and had a Δ14C value of ∼−500‰. The benthic DOC flux of this 14C-depleted, poorly-reactive DOC fraction may represent a source of pre-aged, refractory DOC to the oceans.

Physical control on methanotrophic potential in waters of the Santa Monica Basin, Southern California

Methane concentrations and turnover rates were measured throughout the water column at nine stations in the Santa Monica Basin (SMB), at one station in the San Pedro Basin (SPB), and at one station in the Santa Catalina Basin (SCtB) in July 2007 and September 2009. Methane concentrations were elevated throughout the water column, with subsurface (4–15 nmol L−1), midwater (7.5–100 nmol L−1), and bottom‐water (5–242 nmol L−1) maxima. The SMB water column was divided into four depth‐dependent regimes, based on temperature‐salinity relationships and patterns in methane concentration and fractional turnover rate. We propose that distinct physical controls on methane dynamics distinguish these regimes. In the upper water column, methanotrophic activity appears to be controlled by borderland‐scale, seasonal fluctuations in circulation. The midwaters represent a transition regime between northern‐sourced shallow waters and southern‐sourced deep waters, wherein methanotrophic activity decreases with depth to its minimum at 500 m, coinciding with the midwater methane maxima. Below 500 m, methanotrophic activity increases to the sill depth of 737 m, coincident with increasingly restricted circulation of water. Bottom waters are restricted from through‐basin circulation. These waters were oxygen depleted, but methane input sustained an active psychrotolerant methanotrophic community and had some of the fastest aerobic methane oxidation rates yet reported in the marine environment.

Nitrogen cycling within suboxic and anoxic sediments from the continental margin of Western North America

Here we report rates of benthic nitrogen (N) cycling and assess controls on biological NO 3 − sequestration and transport in sediments underlying oxygen deficient regions of the ocean ranging from anoxic/suboxic ([O 2] of 0–2 μM) to more oxic (57 μM [O 2]) conditions. N mass balances were constructed based on benthic fluxes (N 2, NH 4 + and NO 3 −) and pore water profiles (N 2, NO 3 −, NO 2, N 2O, Fe and HS −) at sites in the Southern California Borderland and the Mexican Margin. Fluxes across the sediment-water interface for N 2, NO 3 − and NH 4 + were determined directly by whole core incubations, and fluxes of N 2 were also determined by modeling mm-scale pore water profiles. Estimates of the N 2 flux by these two methods agree to ± 50%, thereby establishing a range of net N 2 production in these settings. The average N 2 efflux was four times larger at a site with high pore water H 2S concentrations (Soledad Basin 3.14 ± 1.10 mmol N m − 2 day − 1 ) compared to an iron-rich site (Santa Monica Basin 0.74 ± 0.21 mmol N m − 2 day − 1 ) despite both sites having comparable NO 3 − uptake fluxes (− 0.93 ± 0.14 vs. − 0.82 ± 0.08 mmol N m − 2 day − 1 respectively). Pore water profiles from both sulfidic and iron-rich sites reveal subsurface maxima in NO 3 −, NO 2 −, and N 2O that are likely caused by the presence of NO 3 − sequestered by infaunal microbiota. In Soledad Basin, the sequestered pool of microbial NO 3 − contributes to NH 4 + production via DNRA resulting in an NH 4 + efflux (2.66 ± 0.52 mmol N m − 2 day − 1 ) to the overlying water. This flux exceeds the rate of NH 4 + production by C org degradation by 10 times. At the suboxic sites, a total N balance can only be achieved if the flux of NO 3 − into sediments is composed of two components: diffusive and biologically mediated transport. The more oxic site shows no evidence of a sequestered microbial NO 3 − pool and diffusive fluxes can account for all N transformations. Core incubations do not capture the total amount of NO 3 − uptake where biological transport is important as they do not account for NO 3 − sequestered prior to the start of the incubation. Pore water N 2O concentrations of up to 500 nM in sub-surface sediments greatly exceed the background concentration (7 nM) and are likely generated by the metabolic reduction of the intracellular nitrate pool, however, there was no measurable efflux of N 2O from sediments to the overlying water. Biological NO 3 − transport is a ubiquitous process in suboxic and anoxic sediments, however the magnitude of its importance appears to be linked to the presence of dissolved iron or sulfide in the pore waters.