Matthew D. McCarthy

University of California, Santa Cruz, Santa Cruz, California, United States

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Publications (19)98.93 Total impact

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    ABSTRACT: [1] One of the greatest drivers of historical nutrient and sediment transport into the Gulf of Mexico is the unprecedented scale and intensity of land-use change in the Mississippi River Basin. These landscape changes are linked to enhanced fluxes of carbon and nitrogen pollution from the Mississippi River, and persistent eutrophication and hypoxia in the northern Gulf of Mexico. Increased terrestrial runoff is one hypothesis for recent enrichment in bulk nitrogen isotope (δ15N) values, a tracer for nutrient source, observed in a Gulf of Mexico deep-sea coral record. However, unambiguously linking anthropogenic land-use change to whole scale shifts in downstream Gulf of Mexico biogeochemical cycles is difficult. Here we present a novel approach, coupling a new tracer of agro-industrialization to a multi-proxy record of nutrient loading in long-lived deep-sea corals collected in the Gulf of Mexico. We found that coral bulk δ15N values are enriched over the last 150-200 years relative to the last millennia, and compound-specific amino acid δ15N data indicates a strong increase in baseline δ15N of nitrate as the primary cause. Coral rhenium (Re) values are also strongly elevated during this period, suggesting 34% of Re is of anthropogenic origin, consistent with Re enrichment in major world rivers. However, there are no pre-anthropogenic measurements of Re to confirm this observation. For the first time, an unprecedented record of natural and anthropogenic Re variability is documented through coral Re records. Taken together, these novel proxies link upstream changes in water quality to impacts on the deep-sea coral ecosystem.
    Global Biogeochemical Cycles. 01/2014;
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    ABSTRACT: The nitrogen (N) isotopic composition (δ15N) of bulk sedimentary N (δ15Nbulk) is a common tool for studying past biogeochemical cycling in the paleoceanographic record. Empirical evidence suggests that natural fluctuations in the δ15N of surface nutrient N are reflected in the δ15N of exported planktonic biomass and in sedimentary δ15Nbulk. However, δ15Nbulk is an analysis of total combustible sedimentary N, and therefore also includes mixtures of N sources and/or selective removal or preservation of N-containing compounds. Compound-specific nitrogen isotope analyses of individual amino acids (δ15NAA) are novel measurements with the potential to decouple δ15N changes in nutrient N from trophic effects, two main processes that can influence δ15Nbulk records. As a proof of concept study to examine how δ15NAA can be applied in marine sedimentary systems, we compare the δ15NAA signatures of surface and sinking POM sources with shallow surface sediments from the Santa Barbara Basin, a sub-oxic depositional environmental that exhibits excellent preservation of sedimentary organic matter. Our results demonstrate that δ15NAA signatures of both planktonic biomass and sinking POM are well preserved in such surface sediments. However, we also observed an unexpected inverse correlation between δ15N value of phenylalanine (δ15NPhe; the best AA proxy for N isotopic value at the base of the food web) and calculated trophic position. We used a simple N isotope mass balance model to confirm that over long time scales, δ15NPhe values should in fact be directly dependent on shifts in ecosystem trophic position. While this result may appear incongruent with current applications of δ15NAA in food webs, it is consistent with expectations that paleoarchives will integrate N dynamics over much longer timescales. We therefore propose that for paleoceanographic applications, key δ15NAA parameters are ecosystem trophic position, which determines relative partitioning of 15N into source AA versus trophic AA pools, and the integrated δ15NAA of all common protein AA (δ15NTHAA), which serves as a proxy for the δ15N of nutrient N. Together, we suggest that these can provide a coupled picture of regime shifts in planktonic ecosystem structure, δ15N at the base of food webs, and possibly additional information about nutrient dynamics.
    Geochimica et Cosmochimica Acta 01/2014; 142:553–569. · 3.88 Impact Factor
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    ABSTRACT: Deep-sea proteinaceous corals represent high-resolution paleoarchives, extending biogeochemical time series far beyond recent instrumental data. While recent studies have applied compound specific amino acid δ15N (δ15N-AA) measurements of their organic skeletal layers to investigate Holocene nitrogen cycling, potential applications of amino acid δ13C (δ13C-AA) in proteinaceous corals have not yet been examined. Here we developed δ13C-AA analysis in deep-sea bamboo coral (Isidella sp.) from the Monterey Canyon to reconstruct exported primary production over an ~ 80 year record. Preserved deep-sea coral essential amino acid δ13C-AA patterns (δ13C-EAA) closely matched those expected from natural and cultured phytoplankton, supporting the hypothesis that deep-sea coral δ13C-EAA values represent unaltered signatures of exported primary production sources. The coral bulk δ13C record showed cyclic 0.5‰ variations over the last century, with a shift to lower δ13C values in the early 1960s. Variations in coral δ13C-EAA values closely followed bulk δ13C signatures, although both the range and the magnitude of change in the bulk δ13C record were highly attenuated compared to the δ13C-EAA record. Our results indicate that δ13C-EAA in proteinaceous corals represent a new, direct proxy for δ13C in primary production that is more sensitive and accurate than bulk δ13C. To test this idea, we used existing phytoplankton δ13C-AA data to calculate an offset between bulk δ13C and δ13C-EAA. When applied to our data, a reconstructed record of δ13C values for exported organic matter was consistent with regional phytoplankton dynamics and expected trophic transfer effects, suggesting significant AA resynthesis only in the non-essential AA pool. Together, these results indicate that δ13C-EAA in deep-sea proteinaceous corals provide a powerful new long-term, high resolution tool for investigating variations in exported primary production and biogeochemistry.
    Marine Chemistry 01/2014; · 3.00 Impact Factor
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    ABSTRACT: Climatic variation alters biochemical and ecological processes, but it is difficult both to quantify the magnitude of such changes, and to differentiate long-term shifts from inter-annual variability. Here, we simultaneously quantify decade-scale isotopic variability at the lowest and highest trophic positions in the offshore California Current System (CCS) by measuring δ15N and δ13C values of amino acids in a top predator, the sperm whale (Physeter macrocephalus). Using a time series of skin tissue samples as a biological archive, isotopic records from individual amino acids (AAs) can reveal the proximate factors driving a temporal decline we observed in bulk isotope values (a decline of ≥1 ‰) by decoupling changes in primary producer isotope values from those linked to the trophic position of this toothed whale. A continuous decline in baseline (i.e., primary producer) δ15N and δ13C values was observed from 1993 to 2005 (a decrease of ∼4‰ for δ15N source-AAs and 3‰ for δ13C essential-AAs), while the trophic position of whales was variable over time and it did not exhibit directional trends. The baseline δ15N and δ13C shifts suggest rapid ongoing changes in the carbon and nitrogen biogeochemical cycling in the offshore CCS, potentially occurring at faster rates than long-term shifts observed elsewhere in the Pacific. While the mechanisms forcing these biogeochemical shifts remain to be determined, our data suggest possible links to natural climate variability, and also corresponding shifts in surface nutrient availability. Our study demonstrates that isotopic analysis of individual amino acids from a top marine mammal predator can be a powerful new approach to reconstructing temporal variation in both biochemical cycling and trophic structure.
    PLoS ONE 01/2014; 9(10):e110355. · 3.53 Impact Factor
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    Natasha L Vokhshoori, Matthew D McCarthy
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    ABSTRACT: We explored δ15N compound-specific amino acid isotope data (CSI-AA) in filter-feeding intertidal mussels (Mytilus californianus) as a new approach to construct integrated isoscapes of coastal primary production. We examined spatial δ15N gradients in the California Upwelling Ecosystem (CUE), determining bulk δ15N values of mussel tissue from 28 sites between Port Orford, Oregon and La Jolla, California, and applying CSI-AA at selected sites to decouple trophic effects from isotopic values at the base of the food web. Bulk δ15N values showed a strong linear trend with latitude, increasing from North to South (from ∼7‰ to ∼12‰, R2 = 0.759). In contrast, CSI-AA trophic position estimates showed no correlation with latitude. The δ15N trend is therefore most consistent with a baseline δ15N gradient, likely due to the mixing of two source waters: low δ15N nitrate from the southward flowing surface California Current, and the northward transport of the California Undercurrent (CUC), with15N-enriched nitrate. This interpretation is strongly supported by a similar linear gradient in δ15N values of phenylalanine (δ15NPhe), the best AA proxy for baseline δ15N values. We hypothesize δ15NPhe values in intertidal mussels can approximate annual integrated δ15N values of coastal phytoplankton primary production. We therefore used δ15NPhe values to generate the first compound-specific nitrogen isoscape for the coastal Northeast Pacific, which indicates a remarkably linear gradient in coastal primary production δ15N values. We propose that δ15NPhe isoscapes derived from filter feeders can directly characterize baseline δ15N values across major biochemical provinces, with potential applications for understanding migratory and feeding patterns of top predators, monitoring effects of climate change, and study of paleo- archives.
    PLoS ONE 01/2014; 9(6):e98087. · 3.53 Impact Factor
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    ABSTRACT: The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean. Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters. It is thought that this apparent paradox can be explained by a shift in plankton community structure from mostly eukaryotes to mostly nitrogen-fixing prokaryotes. It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend. Here we analyse records of bulk and amino-acid-specific (15)N/(14)N isotopic ratios (δ(15)N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago; these isotopic records serve as a proxy for the source of nitrogen-supported export production through time. We find that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, δ(15)N decreases between 1850 and the present. The total shift in δ(15)N of -2 per mil over this period is comparable to the total change in global mean sedimentary δ(15)N across the Pleistocene-Holocene transition, but it is happening an order of magnitude faster. We use a steady-state model and find that the isotopic mass balance between nitrate and nitrogen fixation implies a 17 to 27 per cent increase in nitrogen fixation over this time period. A comparison with independent records suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.
    Nature 12/2013; · 38.60 Impact Factor
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    ABSTRACT: Compound-specific isotope analysis of individual amino acids (CSI-AA) is a powerful new tool for tracing nitrogen (N) source and transformation in biogeochemical cycles. Specifically, the δ(15) N value of phenylalanine (δ(15) NPhe ) represents an increasingly used proxy for source δ(15) N signatures, with particular promise for paleoceanographic applications. However, current derivatization/gas chromatography methods require expensive and relatively uncommon instrumentation, and have relatively low precision, making many potential applications impractical. A new offline approach has been developed for high-precision δ(15) N measurements of amino acids (δ(15) NAA ), optimized for δ(15) NPhe values. Amino acids (AAs) are first purified via high-pressure liquid chromatography (HPLC), using a mixed-phase column and automated fraction collection. The δ(15) N values are determined via offline elemental analyzer-isotope ratio mass spectrometry (EA-IRMS). The combined HPLC/EA-IRMS method separated most protein AAs with sufficient resolution to obtain accurate δ(15) N values, despite significant intra-peak isotopic fractionation. For δ(15) NPhe values, the precision was ±0.16‰ for standards, 4× better than gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS; ±0.64‰). We also compared a δ(15) NPhe paleo-record from a deep-sea bamboo coral from Monterey Bay, CA, USA, using our method versus GC/C/IRMS. The two methods produced equivalent δ(15) NPhe values within error; however, the δ(15) NPhe values from HPLC/EA-IRMS had approximately twice the precision of GC/C/IRMS (average stdev of 0.27‰ ± 0.14‰ vs 0.60‰ ± 0.20‰, respectively). These results demonstrate that offline HPLC represents a viable alternative to traditional GC/C/IMRS for δ(15) NAA measurement. HPLC/EA-IRMS is more precise and widely available, and therefore useful in applications requiring increased precision for data interpretation (e.g. δ(15) N paleoproxies). Copyright © 2013 John Wiley & Sons, Ltd.
    Rapid Communications in Mass Spectrometry 11/2013; 27(21):2327-2337. · 2.51 Impact Factor
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    Rocio I Ruiz-Cooley, Lisa T Ballance, Matthew D McCarthy
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    ABSTRACT: Coincident with climate shifts and anthropogenic perturbations, the highly voracious jumbo squid Dosidicus gigas reached unprecedented northern latitudes along the NE Pacific margin post 1997-98. The physical or biological drivers of this expansion, as well as its ecological consequences remain unknown. Here, novel analysis from both bulk tissues and individual amino acids (Phenylalanine; Phe and Glutamic acid; Glu) in both gladii and muscle of D. gigas captured in the Northern California Current System (NCCS) documents for the first time multiple geographic origins and migration. Phe δ(15)N values, a proxy for habitat baseline δ(15)N values, confirm at least three different geographic origins that were initially detected by highly variable bulk δ(15)N values in gladii for squid at small sizes (<30 cm gladii length). In contrast, bulk δ(15)N values from gladii of large squid (>60 cm) converged, indicating feeding in a common ecosystem. The strong latitudinal gradient in Phe δ(15)N values from composite muscle samples further confirmed residency at a point in time for large squid in the NCCS. These results contrast with previous ideas, and indicate that small squid are highly migratory, move into the NCCS from two or more distinct geographic origins, and use this ecosystem mainly for feeding. These results represent the first direct information on the origins, immigration and habitat use of this key "invasive" predator in the NCCS, with wide implications for understanding both the mechanisms of periodic D. gigas population range expansions, and effects on ecosystem trophic structure.
    PLoS ONE 01/2013; 8(3):e59651. · 3.53 Impact Factor
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    ABSTRACT: Tracing the origin of nutrients is a fundamental goal of food web research but methodological issues associated with current research techniques such as using stable isotope ratios of bulk tissue can lead to confounding results. We investigated whether naturally occurring δ(13)C patterns among amino acids (δ(13)CAA) could distinguish between multiple aquatic and terrestrial primary production sources. We found that δ(13)CAA patterns in contrast to bulk δ(13)C values distinguished between carbon derived from algae, seagrass, terrestrial plants, bacteria and fungi. Furthermore, we showed for two aquatic producers that their δ(13)CAA patterns were largely unaffected by different environmental conditions despite substantial shifts in bulk δ(13)C values. The potential of assessing the major carbon sources at the base of the food web was demonstrated for freshwater, pelagic, and estuarine consumers; consumer δ(13)C patterns of essential amino acids largely matched those of the dominant primary producers in each system. Since amino acids make up about half of organismal carbon, source diagnostic isotope fingerprints can be used as a new complementary approach to overcome some of the limitations of variable source bulk isotope values commonly encountered in estuarine areas and other complex environments with mixed aquatic and terrestrial inputs.
    PLoS ONE 01/2013; 8(9):e73441. · 3.53 Impact Factor
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    Brett D Walker, Matthew D McCarthy
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    ABSTRACT: We report the interseasonal variation in bulk elemental and stable isotopic (δ13C, δ15N) composition of dissolved and particulate organic matter (DOM, POM) during a high-resolution time series (2007–2009) on the Big Sur coast. In addition to interseasonal variations, we explore the relationships between physical size and reactivity (i.e., composition) of exportable organic matter (OM) pools, and characterize the elemental and isotopic composition of size-fractionated POM and DOM pools within this potentially high-nutrient, low-chlorophyll (HNLC) California upwelling region. Average POM concentrations were low (< 5.2 mmol C L-1 and < 0.7 mmol N L-1), and all OM pools had low C : N ratios (DOM average = 15.0 and POM < 7.8)—indicating that this upwelling center may represent an important source of N-rich material to offshore environments. Seasonal ‘‘excess’’ dissolved organic carbon (DOC) production was significant (~ 30–80 mmol L-1 DOC); however, dissolved organic nitrogen (DON) production and cycling was largely decoupled from both DOC and physical processes. Overall, our results are consistent with this region representing an ‘‘HNLC upwelling system.’’ The distinct bulk elemental and isotopic OM composition and seasonality vs. that of high-productivity upwelling regions highlights the need to better understand the biogeochemical diversity of upwelling systems. Finally, we observed a quantifiable size–composition relationship across both POM and DOM size classes, perhaps representing a powerful new tool for modeling N vs. C fluxes in ocean biogeochemical cycles.
    Limnology and Oceanography 01/2012; 57(6):1757–1774. · 3.62 Impact Factor
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    ABSTRACT: Despite the importance of the nitrogen (N) cycle on marine productivity, little is known about variability in N sources and cycling in the ocean in relation to natural and anthropogenic climate change. Beyond the last few decades of scientific observation, knowledge depends largely on proxy records derived from nitrogen stable isotopes (δ(15)N) preserved in sediments and other bioarchives. Traditional bulk δ(15)N measurements, however, represent the combined influence of N source and subsequent trophic transfers, often confounding environmental interpretation. Recently, compound-specific analysis of individual amino acids (δ(15)N-AA) has been shown as a means to deconvolve trophic level versus N source effects on the δ(15)N variability of bulk organic matter. Here, we demonstrate the first use of δ(15)N-AA in a paleoceanographic study, through analysis of annually secreted growth rings preserved in the organic endoskeletons of deep-sea gorgonian corals. In the Northwest Atlantic off Nova Scotia, coral δ(15)N is correlated with increasing presence of subtropical versus subpolar slope waters over the twentieth century. By using the new δ(15)N-AA approach to control for variable trophic processing, we are able to interpret coral bulk δ(15)N values as a proxy for nitrate source and, hence, slope water source partitioning. We conclude that the persistence of the warm, nutrient-rich regime since the early 1970s is largely unique in the context of the last approximately 1,800 yr. This evidence suggests that nutrient variability in this region is coordinated with recent changes in global climate and underscores the broad potential of δ(15)N-AA for paleoceanographic studies of the marine N cycle.
    Proceedings of the National Academy of Sciences 01/2011; 108(3):1011-5. · 9.81 Impact Factor
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    ABSTRACT: Stable carbon and nitrogen isotope ratios (δ13C and δ15N) of serum, red blood cells (RBC), muscle, and blubber were measured in captive and wild northeast Pacific harbor seals (Phoca vitulina richardii) at three coastal California sites (San Francisco Bay, Tomales Bay, and Channel Islands). Trophic discrimination factors (ΔTissue-Diet) were calculated for captive seals and then applied in wild counterparts in each habitat to estimate trophic position and feeding behavior. Trophic discrimination factors for δ15N of serum (+3.8‰), lipid-extracted muscle (+1.6‰), and lipid-blubber (+6.5‰) are proposed to determine trophic position. An offset between RBC and serum of +0.3‰ for δ13C and −0.6‰ for δ15N was observed, which is consistent with previous research. Specifically, weaner seals (<1 yr) had large offsets, suggesting strong trophic position shifts during this life stage. Isotopic values indicated an average trophic position of 3.6 at both San Francisco Bay and Tomales Bay and 4.2 at Channel Islands. Isotopic means were strongly dependent on age class and also suggested that mean diet composition varies considerably between all locations. Together, these data indicate that isotopic composition of blood fractions can be an effective approach to estimate trophic position and dietary behavior in wild pinnipeds.
    Marine Mammal Science 01/2011; · 2.13 Impact Factor
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    ABSTRACT: Hydrothermal fluids circulate through extensive areas of the upper oceanic crust. Most hydrothermal circulation occurs on ridge flanks, where low-temperature fluids flow through porous basalts. These fluids contain variable levels of dissolved organic carbon, but the source and composition of this carbon are uncertain. Here, we report Δ14C and δ13C measurements of dissolved organic carbon in ridge-flank and on-axis hydrothermal fluids sampled from the Juan de Fuca Ridge. Dissolved organic carbon from two independent ridge-flank sites was characterized by low δ13C and Δ14C values. The δ13C values ranged from -26 to -35‰, and were consistent with a chemoautotrophic origin. The 14C ages of the dissolved organic carbon ranged from 11,800 to 14,400 years before present, revealing that the carbon was around three times older than dissolved organics in the deep ocean. The Δ14C values of the ridge-flank dissolved organic matter also corresponded closely to those of dissolved inorganic carbon in the same fluid samples. Taken together, the data suggest that chemosynthetic crustal microbial communities synthesize dissolved organic carbon from inorganic carbon in ridge-flank fluids. We suggest that ridge-flank circulation may support an indigenous biosphere extensive enough to export substantial fixed carbon, with distinct isotopic and probably compositional character, to the overlying ocean.
    Nature Geoscience 01/2010; · 11.67 Impact Factor
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    ABSTRACT: We analyzed stable carbon (δ13C) and radiocarbon (Δ14C) isotopes of ocean crustal fluid samples from two low-temperature environments on and near the Juan de Fuca Ridge (JDFR), a seafloor spreading center in the northeastern Pacific Ocean. The major goals of this work were to resolve relative dissolved inorganic carbon (DIC) sources and removal processes, and characterize the isotopic signatures of DIC vented to the overlying ocean. DIC was isolated from diffuse vents on the Main Endeavour Field (MEF), on zero age seafloor, and from two ridge-flank sites located 100 km to the east, on 3.5 Ma seafloor; the Baby Bare outcrop and Ocean Drilling Program (ODP) Hole 1026B. Low-temperature MEF fluids were enriched in DIC (3.13 to 5.51 mmol kg− 1) relative to background seawater (2.6 mmol kg− 1), and their Δ14C and δ13C values are consistent with simple two-endmember mixing of pure high-temperature (≥ 300 °C) hydrothermal fluid and bottom seawater (secondary recharge). These data suggest that no major sedimentary or biological DIC sources are present, however our results do not preclude a minor sedimentary influence on low-temperature MEF vent fluids. DIC Δ14C and δ13C values of ridge-flank fluids from this area are consistent with an off-axis recharge source, followed by water–rock interaction at moderate temperatures (60–70 °C) during flow through basement. An observed offset in radiocarbon ages between fluids from Baby Bare outcrop and Hole 1026B (∼ 1100 yr) is consistent with crustal flow from south to north, at rates similar to those inferred from other geochemical and thermal tracers. The ridge-flank hydrothermal fluids are strongly depleted in DIC and δ13C relative to bottom seawater, suggesting more extensive carbon removal in this setting (∼ 5.7 × 1012 mol C yr− 1) than has been previously suggested. DIC isotopic depletion is consistent with carbonate vein precipitation in conjunction with a minor addition of CO2 from basalt vesicles, and suggests that ridge-flank systems may be an important sink for seawater inorganic carbon, and comprise an important global reservoir of isotopically depleted and “pre-aged” DIC.
    Marine Chemistry. 01/2008;
  • Leslie A. Roland, Matthew D. McCarthy, Tom Guilderson
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    ABSTRACT: Several recent studies have suggested dramatically different ideas about the source and nature of molecularly uncharacterized organic carbon (MUC) in sinking marine particles (POC). Carbon isotope data coupled with hydrolysis has indicated MUC is lipid-like material, suggesting selective preservation [Hwang, J. and Druffel, E.R.M., 2003. Lipid-like material as the source of the uncharacterized organic carbon in the ocean? Science, 299: 881–884.]. In contrast, NMR-based work has strongly indicated non-selective degradation, with amino acid dominating resistant material [Hedges, J.I. et al., 2001. Evidence for non-selective preservation of organic matter in sinking marine particles. Nature, 409: 801–804.]. This study set out to explore this seeming paradox, and to examine the hypothesis that the nature of MUC may vary strongly between margin and open ocean regions. We examined the coupled elemental, stable and radiocarbon isotopic compositions of three fractions of sinking POC: lipids, acid soluble (AS) material (a proxy for hydrolyzable biomolecules), and acid insoluble (AI) material (a proxy for the MUC). Δ14C and δ13C measurements were made on three time-series of sediment trap samples in widely separate ocean regions: Santa Barbara Basin, Cariaco Basin, and an oceanic site in the Eastern Subtropical Atlantic off Dakar, Africa.Δ14C compositions of AI fractions at all sites indicated substantial contribution by pre-aged marine carbon sources (40–60% of total sinking POC), not derived from direct export of surface productivity. Comparison of δ13C and Δ14C values with coexisting lipid and AS signatures also suggested widely different AI compositions from different ocean environments. AI material in coastal California waters consistently appeared lipid-like, in agreement with several studies near the same region [Hwang, J. and Druffel, E.R.M., 2003. Lipid-like material as the source of the uncharacterized organic carbon in the ocean? Science, 299: 881–884.; Hwang, J., Druffel, E.R.M., Eglinton, T.I. and Repeta, D.J., 2006b. Source(s) and cycling of the nonhydrolyzable organic fraction of oceanic particles. Geochimica et Cosmochimica Acta, 70: 5162–5168.]. In contrast, our oceanic site's AI isotopic signatures were more consistent with non-selective preservation of a range of biochemical classes. AI compositions in Cariaco Basin proved to be variable with time, suggesting a complex and variable mixture of AI sources.These results suggest a major divergence between coastal and oceanic AI sources and composition. Taken together, we propose that the dominant mechanism influencing molecularly uncharacterized material is non-selective preservation during initial water column transit; influenced to varying degrees by subsequent addition of allochthonous and 14C-depleted organic carbon sources. In our coastal margin sites, all AI properties point to resuspended sediment as the most likely source of total uncharacterizable POC. In our more oceanic site, where particles have much longer transit times, incorporation of old DOC into POC, in addition to resuspended sediment, may have a substantial influence on overall Δ14C ages. We propose that major differences between AI compositions in different locations is tied to variation in availability of such pre-aged OC sources, linked to regional oceanographic conditions and strongly influenced by proximity to continental shelves. The strong correlations we observe between %AI vs. %AS composition and elemental and isotopic values also implies that substantial amounts of POC collected via sediment traps in some locations is not exported directly from surface production, but added from other pre-aged reservoirs.
    Marine Chemistry 01/2008; 111:199-213. · 3.00 Impact Factor
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    ABSTRACT: Geochemical sea surface temperature (SST) proxies such as the magnesium to calcium ratio (Mg/Ca) in foraminifera and the alkenone unsaturation index (UK' 37) are becoming widely used in pre-Pleistocene climate records. This study quantitatively compares previously published Mg/Ca and UK' 37 data from Ocean Drilling Program (ODP) site 847 in the eastern equatorial Pacific to assess the utility of these proxies to reconstruct tropical SST over the last 5 Ma. Foraminiferal Mg/Ca-SST calibrations that include a dissolution correction are most appropriate at this location because they provide SST estimates for the youngest sample that are close to modern mean annual SST. The long-term trends in the two records are remarkably similar and confirm a ~3.5°C cooling trend from the early Pliocene warm period to the late Pleistocene noted in previous work. Absolute temperature estimates are similar for both proxies when errors in the dissolution correction used to estimate SST from Mg/Ca are taken into account. Comparing the two SST records at ODP site 847 to other records in the region shows that the eastern equatorial Pacific was 2-4°C warmer during the early Pliocene compared to today.
    Geochemistry Geophysics Geosystems 01/2008; 9. · 2.94 Impact Factor
  • Petra S. Dekens, Ana Christina Ravelo, Matthew D. McCarthy
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    ABSTRACT: Given the importance of upwelling processes to coastal productivity and regional climate, it is critical to study the role of upwelling regions within the context of global climate change. We generated sea surface temperature (SST) records for the last 5 million years in three important upwelling regions: the eastern equatorial Pacific, the California margin, and the Peru margin. Prior to ~3.0 Ma, SSTs at all sites were significantly warmer than today (by 3-9°C), indicating that cold upwelling regions that characterize the modern Pacific Ocean did not exist in the early Pliocene warm period (4.6 to 3.1 Ma), Earth's most recent period of sustained global warmth. Alkenone, phosphorus, and organic carbon mass accumulation rate records indicate that changes in productivity and SST were decoupled and that upwelling of nutrient enriched water occurred even when SSTs were warm during the early Pliocene. Thus the long-term trends in SST are likely explained by changes in the temperature of upwelled water rather than in the strength of upwelling-favorable winds alone. The fact that gradual cooling of upwelling regions began before the onset of significant Northern Hemisphere glaciation provides further evidence that the growth of ice sheets and their influence on atmospheric winds alone can not explain the cooling of upwelling regions. Our results suggest that the long-term average SSTs of upwelling regions are influenced by global changes in the depth and/or temperature of the ventilated thermocline.
    Paleoceanography 01/2007; 22. · 3.30 Impact Factor
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    Matthew D. McCarthy, Ronald Benner, Cindy Lee, Marilyn L. Fogel
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    ABSTRACT: Bulk nitrogen (N) isotope signatures have long been used to investigate organic N source and food web structure in aquatic ecosystems. This paper explores the use of compound-specific δ15N patterns of amino acids (δ15N-AA) as a new tool to examine source and processing history in non-living marine organic matter. We measured δ15N-AA distributions in plankton tows, sinking particulate organic matter (POM), and ultrafiltered dissolved organic matter (UDOM) in the central Pacific Ocean. δ15N-AA patterns in eukaryotic algae and mixed plankton tows closely resemble those previously reported in culture. δ15N differences between individual amino acids (AA) strongly suggest that the sharply divergent δ15N enrichment for different AA with trophic transfer, as first reported by [McClelland, J.W. and Montoya, J.P. (2002) Trophic relationships and the nitrogen isotopic composition of amino acids. Ecology83, 2173–2180], is a general phenomenon. In addition, differences in δ15N of individual AA indicative of trophic transfers are clearly preserved in sinking POM, along with additional changes that may indicate subsequent microbial reworking after incorporation into particles.We propose two internally normalized δ15N proxies that track heterotrophic processes in detrital organic matter. Both are based on isotopic signatures in multiple AA, chosen to minimize potential problems associated with any single compound in degraded materials. A trophic level indicator (ΔTr) is derived from the δ15N difference between selected groups of AA based on their relative enrichment with trophic transfer. We propose that a corresponding measure of the variance within a sub-group of AA (designated ΣV) may indicate total AA resynthesis, and be strongly tied to heterotrophic microbial reworking in detrital materials. Together, we hypothesize that ΔTr and ΣV define a two dimensional trophic “space”, which may simultaneously express relative extent of eukaryotic and bacterial heterotrophic processing.In the equatorial Pacific, ΔTr indicates an average of 1.5–2 trophic transfers between phytoplankton and sinking POM at all depths and locations. The ΣV parameter suggests that substantial variation may exist in bacterial heterotrophic processing between differing regions and time periods. In dissolved material δ15N-AA patterns appear unrelated to those in POM. In contrast to POM, δ15N-AA signatures in UDOM show no clear changes with depth, and suggest that dissolved AA preserved throughout the oceanic water column have undergone few, if any, trophic transfers. Together these data suggest a sharp divide between processing histories, and possibly sources, of particulate vs. dissolved AA.
    Geochimica et Cosmochimica Acta 01/2007; · 3.88 Impact Factor
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    ABSTRACT: The transfer of dissolved organic carbon (DOC) and nitrogen (DON) out of the surface ocean where it is produced to storage in the ocean's interior creates one of the largest reservoirs of reduced carbon and organic nitrogen on earth. In nutrient-depleted surface waters of the oligotrophic ocean, dissolved nitrogenous material is of key importance as a source of fixed nitrogen for heterotrophic organisms. Recent work has increasingly indicated that, contrary to previous ideas, recalcitrant chemical structure is not the central factor underlying the preservation of DOC and DON, leaving the major preservation mechanisms largely unknown. We employ here a stable isotopic approach to examine the metabolic source and transformation signatures imprinted in carbon isotopic fractionation patterns of amino acids, which are the major components of both particulate and dissolved organic nitrogen that can be identified at the molecular level. Compound-specific isotopic signatures from central Pacific particulate and dissolved organic matter indicate a profound difference in processing histories between these two material pools. Sinking particles show a clear imprint of heterotrophic resynthesis and alteration, while the much larger and older dissolved pool retains an unaltered signature of photoautotrophic synthesis, even in samples from the abyssal ocean. In addition, δ13C signatures of enantiomers of alanine (d vs. l) in dissolved materials are indistinguishable. This isotopic data, in light of previously observed abundant d-amino acids in oceanic DOM, suggests that autotrophic prokaryotes may be a main source for dissolved nitrogenous material preserved over long time scales in the sea. Taken together, our results suggest that dissolved organic nitrogen preservation is not predominantly linked to heterotrophic reworking and resynthesis, but instead there exists a non-discriminating and rapid shunt, effectively removing recently formed autotrophic biomolecules from further recycling.
    Marine Chemistry. 01/2004;
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    ABSTRACT: Introduction Particulate organic matter (POM) represents one of the largest dynamic carbon reservoirs in the ocean. Sinking POM is a source of regenerated nutrients that support primary pro-ductivity, hetetrophic life in the subsurface, and ultimately represent the major removal pathway for atmospheric CO 2 (Lee et al. 2005; Suess 1980; Wakeham et al. 1997). Whereas sinking POM is a key component of the ocean carbon cycle, the larger proportion of the total POM pool exists suspended in the water column (Lee et al. 2005; McNichol and Aluwihare 2007; Megens et al. 2002). Whereas suspended and sinking POM are linked to some extent via aggregation/disaggregation processes (Alldredge and Jackson 1995; McCave 1984), POC susp has distinct chemical and isotopic compositions relative to its sinking counterpart (Druffel et al. 1998; Repeta 1984; Wake-ham and Ertel 1988). Microbial loop processes have a large effect on the POC susp pool by mediating carbon, nutrient, and metal transfers between particulate and dissolved phases (Clegg et al. 1991; Shaw et al. 1998). Finally, because POC susp is primarily advected with water masses, its persistence and ulti-mate degradation consists of very different linkages in the ocean carbon cycle in comparison to sinking POM (Bauer and Druffel 1998). Understanding POM susp preservation and degra-dation processes are therefore central to global carbon and nitrogen cycle dynamics. Despite substantial scientific interest in the sources and cycling of POM susp , the collection of sufficient material for detailed organic chemical analyses is challenging. GFF filters are widely used for collection of POM susp , typically allowing material to be isolated from 1–10 L of seawater. While suitable Abstract We describe the construction and testing of a home-built ultrafiltration (UF) system, based on commercially available hollow fiber polysulfone membranes, for isolation of suspended particulate organic matter (POM susp) from large volumes (2000–10,000 L) of ocean water. The overall apparatus consists of two sequential UF steps: a main filtration system (100 L reservoir) driven by a stainless steel centrifugal pump, and a subsequent reduc-tion/diafiltration system (2 L reservoir) driven by a peristaltic pump. The system can be readily assembled using off-the-shelf parts at a fraction of the cost of commercial UF systems. Our system functioned comparably to pre-viously described commercial units. We conducted a series of tests using both surface (21 m) and mesopelagic (674 m) N. Pacific central seawater from ocean pipeline sources at the Natural Energy Laboratory Authority of Hawaii (NELHA), while simultaneously collecting GFF-POM samples. We evaluated flow rates, fouling behavior, carbon and nitrogen recoveries and compositions, and also bacteria and virus retention of ultrafiltered-POM (UPOM) using both 0.1 μm and 500 kilodalton pore size membranes. We also compared composition of UPOM versus GFF-POM, finding clear differences, which also varied between surface and mesopelagic waters. Finally, to evaluate the appeal of large-volume filtrations at NELHA to study central N. Pacific Gyre POM susp , we com-pared our data with offshore station ALOHA.

Publication Stats

148 Citations
98.93 Total Impact Points

Institutions

  • 2007–2014
    • University of California, Santa Cruz
      • Department of Ocean Sciences
      Santa Cruz, California, United States
  • 2013
    • Southwest Fisheries Science Center
      La Jolla, California, United States
    • Christian-Albrechts-Universität zu Kiel
      Kiel, Schleswig-Holstein, Germany
  • 2011
    • Memorial University of Newfoundland
      • Department of Biology
      Saint John, New Brunswick, Canada