Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments

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Highlights

  • Pore water profiles of NO3 and Si(OH)4 were modeled to define remineralization rates of Corg and bSi.

  • Corg remineralization shows a distinct gradient following the path of the NW Amazon River Plume.

  • Highest rates of bSi remineralization occur further up the axis of NW plume progression.

  • Highest fraction of ‘fast’ reactive Corg lies in NW plume axis.

  • Lowest fraction of ‘fast’ reactive Corg lies in path of eastward plume retroflection.

Abstract

The Amazon River Plume delivers freshwater and nutrients to an otherwise oligotrophic western tropical North Atlantic (WTNA) Ocean. Plume waters create conditions favorable for carbon and nitrogen fixation, and blooms of diatoms and their diazotrophic cyanobacterial symbionts have been credited with significant CO2 uptake from the atmosphere. The fate of the carbon, however, has been measured previously by just a few moored or drifting sediment traps, allowing only speculation about the full extent of the plume's impact on carbon flux to the deep sea. Here, we used surface (0.5 m) sediment cores collected throughout the Demerara Slope and Abyssal Plain, at depths ranging from 1800 to 5000 m, to document benthic diagenetic processes indicative of carbon flux. Pore waters were extracted from sediments using both mm- and cm-scale extraction techniques. Profiles of nitrate (NO3) and silicate (Si(OH)4) were modeled with a diffusion-reaction equation to determine particulate organic carbon (POC) degradation and biogenic silica (bSi) remineralization rates. Model output was used to determine the spatial patterns of POC and bSi arrival at the sea floor. Our estimates of POC and Si remineralization fluxes ranged from 0.16 to 1.92 and 0.14 to 1.35 mmol m−2 d−1, respectively. A distinct axis of POC and bSi deposition on the deep sea floor aligned with the NW axis of the plume during peak springtime flood. POC flux showed a gradient along this axis with highest fluxes closest to the river mouth. bSi had a more diffuse zone of deposition and remineralization. The impact of the Amazon plume on benthic fluxes can be detected northward to 10°N and eastward to 47°W, indicating a footprint of nearly 1 million km2. We estimate that 0.15 Tmol C y−1 is remineralized in abyssal sediments underlying waters influenced by the Amazon River. This constitutes a relatively high fraction (~7%) of the estimated C export from the region.; the plume thus has a demonstrable impact on Corg export in the western Atlantic. Benthic fluxes under the plume were comparable to and in some cases greater than those observed in the eastern equatorial Atlantic, the southeastern Atlantic, and the Southern Ocean.

Introduction

Rivers impact the production of carbon in the ocean by providing a source of nutrients as well as trace elements and stratification that can enhance primary production, resulting in net carbon transport from the atmosphere to the deep sea (Eppley and Peterson, 1979, Raymond and Cole, 2003, Smith and Hitchcock, 1994). The Amazon River is responsible for one fifth of the total riverine discharge to the world oceans (Gibbs, 1972), delivering freshwater at a seasonally-variable rate of 120,000–300,000 m3 s−1 (Cooley et al., 2007, Perry et al., 1996). Although this freshwater mixes with ocean water, a plume of lower salinity waters and CDOM can be traced northwest to the Caribbean in the late spring during periods of maximum discharge as well as eastward into the North Atlantic due to the seasonal retroflection of the North Brazil Current (NBC) in the fall (Del Vecchio and Subramaniam, 2004, Froelich et al., 1978, Johns et al., 1998, Muller-Karger et al., 1988, Muller-Karger et al., 1995).

Although the tropical North Atlantic is generally considered to be a net source of CO2 to the atmosphere (Deuser et al., 1988, Mikaloff Fletcher et al., 2007, Takahashi et al., 2002), CO2 uptake has been observed in the region influenced by the Amazon River Plume (Cooley et al., 2007, Cooley and Yager, 2006, Ternon et al., 2000). Subramaniam et al. (2008) hypothesized that the plume supports ecological niches in mesohaline waters (30<sea surface salinity (SSS)<35) where surface waters are depleted in N but contain a relative excess of dissolved Si and P (Shipe et al., 2006, Subramaniam et al., 2008). This combination creates optimal conditions for diatom–diazotroph associations (DDAs), which have been suggested to be an important vector for carbon sequestration and export in this region (Carpenter et al., 1999, Foster et al., 2007, Subramaniam et al., 2008). Recently, Yeung et al. (2012) confirmed that DDA blooms increase carbon export efficiency; however, it is unclear if the blooms also result in an increase in the material reaching the deep ocean. Floating sediment traps deployed at 150 m (Subramaniam et al., 2008) showed a correlation between the presence of DDAs in the water column and mass flux, but this does not tell us how much carbon reaches the sea floor. A moored trap deployed at 3200 m at 13.22°N, 41.68°E (Deuser et al., 1988), showed a relationship between higher primary productivity and higher particle flux, but it is unknown if DDAs were responsible for the blooms. We have adopted the approach of using the sediments as ‘the ultimate sediment trap’ (Herman et al., 2001, Jahnke, 1990, Reimers et al., 1992) to map and quantify the export of carbon and biogenic Si to the deep sea in the area of the WTNA influenced by the Amazon Plume. Although only a small fraction of the C and bSi exported from surface waters reaches the sea floor at 4500 m, in the absence of significant sediment redistribution (focusing), there should be a benthic signal of C input that relates to production and export.

The majority of work on marine sediments and particulate organic matter in the area of the Amazon Plume has been limited to the river mouth and the adjacent continental shelf (Aller and Blair, 2006, Aller et al., 1998, Aller et al., 2004, Blair et al., 2004, Druffel, 2005, Hedges et al., 1986, Keil et al., 1997, Kineke et al., 1996, Kuehl et al., 1986, Kuehl et al., 1996, Nittrouer and DeMaster, 1996). Our work includes sites 500–1200 km from the Amazon River mouth and focuses primarily on deep-sea (>3500 m) sites in the open ocean. Pore water studies on deep-sea sediments from the Amazon Fan (Kasten et al., 1998, Kasten et al., 2003, Schlünz et al., 1999, Schulz et al., 1994) and the Ceara Rise (Martin and Sayles, 1996, Martin and Sayles, 2006, Wenzhöfer and Glud, 2002) serve as an excellent reference to our study insofar as today these areas are subject to little direct influence from the river plume.

For many open ocean environments, the organic matter reaching the sea floor is nearly completely remineralized during early diagenesis (Bender and Heggie, 1984, Burdige, 2007, Canfield, 1994, Hedges and Keil, 1995, Premuzic et al., 1982), hence the oxidation flux should closely approximate the deposition flux. We measured and modeled pore water NO3 profiles and interpreted their shape as a proxy for oxygen utilization and carbon oxidation (Goloway and Bender, 1982, Martin et al., 1991, Martin and Sayles, 1996, Martin and Sayles, 2006). This approach has a benefit over measurements of oxygen gradients, which are sometimes subject to artifacts due to core recovery (Wenzhöfer et al., 2001). Nitrate profiles, as a proxy of carbon remineralization, are indicative of where deposition is occurring and we compare this ‘footprint’ to the location of the river plume in the surface ocean. We also calculate the flux of dissolved Si to provide an indication of the importance of biogenic silica export throughout the region.

Section snippets

Study area and methods

We present data from the ANACONDAS (Amazon iNfluence on the Atlantic: CarbOn export from Nitrogen fixation by DiAtom Symbioses) project, which consisted of three cruises throughout the WTNA aboard the R/V Knorr (May–June 2010), the R/V Melville (September–October 2011), and the R/V Atlantis (July 2012). We collected sediment cores from 1800–5044 m at 32 sites across the Demerara Slope/Abyssal Plain (Fig. 1), using a multi-corer with 9.8 cm (ID) diameter core tubes (Barnett et al., 1984) (Table 1

Dissolved Si

WCS defines pore water gradients near the SWI but some pore water consituents may be subject to squeezing artifacts (Berelson et al., 1990, Sayles et al., 1996). Because of this effect and because the coarse resolution of Rhizon sampling do not define the curvature in dissolved Si as well as the WCS (McManus et al., 1995), only the top 1 cm of WCS silica profiles was used to determine the flux across the SWI. The data was fit with either a simple polynomial:C=C0+m1x+m2x2or an exponential

Porosity and solid phase analyses

Porosity generally showed a steady decline with depth. The porosity at the SWI determined from the fit to Eqs. (1), (2) ranged from 0.90 to 0.97 (Table 4) and declined to values less than 0.75. Parameters generated from the porosity fitting equations are listed in Supplemental Table ST-1.

The average surface sediment C:N ratio from 9 stations throughout the study region was 6.9±0.6 (Table 2). This value, combined with the Anderson and Sarmiento (1994) C:O2 ratio, yielded a γ of 0.10 (Eq. (6)).

Pore water Si profiles and fluxes

Spatial patterns of benthic fluxes

The sediment footprint of carbon oxidation reveals steep gradients over a much narrower area than the area of the river-influenced plume salinity (Fig. 7). The flux attenuation across this feature is striking, decreasing to the east by 1.5 mmol m−2 d−1 (nearly a factor of 10) and by a similar magnitude to the N and NW. Off the axis of the high flux ‘tongue’, fluxes are fairly low and are generally consistent with values from the equatorial and low latitude central Pacific (Hammond et al., 1996).

Conclusions

The Amazon River Plume leaves a distinct footprint of Corg and bSi deposition and remineralization in the underlying deep sea sediments as far as 1200 km from the river mouth. There is an axis of Corg and bSi deposition that mirrors the axis of the freshwater plume as it travels NW from 5°N to 50°W. Both Corg and bSi remineralization rates are attenuated to the NW and E. Benthic carbon oxidation fluxes ranged from 0.16 to 1.92 mmol m−2 d−1 throughout the study region, with the highest fluxes

Acknowledgments

NSF Grant #OCE-0934073 supported the contributions of WB, LC and NR, and #0929339 and #1029889 supported the contributions of JM. PY was supported by OCE-0934095 and the Gordon and Betty Moore Foundation. We would like to thank the three anonymous reviewers and the DSR editors whose input improved this manuscript. We are thankful to the captains and crews of the R/V Knorr, R/V Melville, and R/V Atlantis. We would also like to acknowledge the members of the at-sea and shore-based science party

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    Present address: University of Akron, Department of Geosciences, Akron, OH 44325-4101, United States.

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