Sources of carbon to suspended particulate organic matter in the northern Gulf of Mexico

Suspended particulate organic carbon (POC susp ) in the Gulf of Mexico is unique compared to other seas and oceans. In addition to surface primary production, isotopic analysis indicates that microbial cycling of oil and riverine inputs are primary sources of carbon to POC susp in the Gulf. To characterize POC susp from seep sites and non-seep north central Gulf (NCG) sites potentially affected by the Deepwater Horizon (DWH) spill, we analyzed 277 and 123 samples for δ 13 C and Δ 14 C signatures, respectively. Depth, partitioned into euphotic (<300 m) and deep (>300 m), was the main driver of spatial δ 13 C differences, with deep depths exhibiting 13 C depletion. Both deep depths and proximity to sources of natural seepage resulted in 14 C depletion. A two-endmember mixing model based on Δ 14 C indicated that sources to POC susp were 14–29% fossil carbon at NCG sites and 19–57% at seep sites, with the balance being modern surface production. A six-component Bayesian mixing model MixSIAR, using both 13 C and 14 C, suggested that riverine inputs were an important carbon source to POC susp contributing 34–46%. The influence of seeps was localized. Below the euphotic zone at seep sites, 46 ± 5% (n = 9) of the carbon in POC susp was derived from envi-ronmentally degraded, transformed oil; away from seeps, transformed oil contributed 15 ± 4% (n = 39). We hypothesized that, at NCG sites removed from hydrocarbon seep sources, isotopic signatures would be depleted following the spill and then shift towards background-like enriched values over time. At deep depths we observed decreasing Δ 14 C signatures in POC susp from 2010 to 2012, followed by isotopic enrichment from 2012 to 2014 and a subsequent recovery rate of 159‰ per year, consistent with this hypothesis and with biodegraded material from DWH hydrocarbons contributing to POC susp .


Introduction
The introduction of fossil hydrocarbon-derived material, whether by anthropogenic inputs or natural seepage, provides a unique source of carbon source to the sea. In most areas of the Atlantic and Pacific oceans, surface primary production is the main carbon source to the deep ocean. Typically, only about 1% of the carbon fixed at the surface reaches the deep seafloor. Along the way, most of the organic matter is consumed and degraded, exchanging between the different carbon pools in the water columndissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and sinking and suspended particulate organic carbon (POC sink , POC susp ), before the residual amount finally reaches the seafloor.
Of these pools, POC susp is uniquely capable of providing insights into the sources of carbon to the water column and those fueling the microbial loop. The small particle size and relatively short residence time that characterize POC susp make it more sensitive than other carbon pools to recording variations in inputs. The different carbon pools can be defined operationally by size. The smallest end of this size continuum is DOC, which we define as organic carbon that passes through a filter of 0.7-µm pore size. POC susp is any organic matter collected on the 0.7-µm filter, while POC sink is comprised of those particles typically larger than 50 µm (Deuser, 1986). Due to the small difference in size, DOC and POC susp are more similar to each other chemically than to POC sink (Druffel et al., 1996). As DOC is partially controlled by microbial processes, the DOC-POC susp connection provides a link between microbial processes and the larger particles that can move carbon up the food chain Cherrier et al., 2014). Linkage with the microbial

RESEARCH ARTICLE
Sources of carbon to suspended particulate organic matter in the northern Gulf of Mexico loop has also been observed in the microbial uptake of dissolved inorganic nitrogen and possible methanodiazotrophy through δ 15 N isotope analysis (Montoya et al., 1990;Fernandez et al., 2016).
The residence times of DOC, POC sink , and POC susp range widely, resulting in different degrees of sensitivity of each carbon pool to different inputs. DOC is the second largest carbon reservoir in the ocean, amounting to about 650 Pg of mostly recalcitrant carbon, with a residence time of 1000-6000 years (Williams and Druffel, 1987). The concentrations of POC susp and POC sink in the water are much lower than DOC, but the flux of POC sink through the water column is much greater than POC susp , with a residence time of about a month (Deuser, 1986). POC sink is determined primarily by surface phytoplankton production , which draws on the DIC pool, the largest carbon reservoir in the ocean at 38,000 Pg (Hansell and Carlson, 2014). POC susp floats in the water for 5-10 years (Bacon and Anderson, 1982). Its low concentrations in the open ocean, typically from less than 10 µM C in surface waters to about 1 µM C at depths below 500 m (McNichol and Aluwihare, 2007), and short residence times increase the sensitivity of this pool to other carbon sources, although the DOC and POC sink pools have also been observed to reflect variations in carbon input associated with the Gulf of Mexico (GOM) Deepwater Horizon (DWH) spill (Yan et al., 2016;Walker et al., 2017;Chanton et al., 2018;Geiring et al., 2018).
Studies in the late 1980s and early 1990s used stable and radiocarbon isotope analysis of POC to determine sources of oceanic particulates. Druffel et al. (1992Druffel et al. ( , 1996Druffel et al. ( , 2003 analyzed stable and radiocarbon isotopes of particulates from the Sargasso Sea and central North Pacific which indicated that POC sink and POC susp are derived primarily from surface phytoplankton production. POC susp is enriched in radiocarbon from being formed at the surface and becomes more depleted with depth. In the GOM, δ 13 C values for surface production range from -20 to -22‰ (Chanton and Lewis, 2002), and, at the time of the oil spill, Δ 14 C values for surface production ranged from 39 to 41‰ .
In marginal seas, the sources of POC susp can vary. Bauer et al. (2002) found highly depleted δ 13 C and Δ 14 C values for POC susp at depth along the Mid-Atlantic Bight (MAB). The radiocarbon depletion near the seafloor of POC susp has been attributed to resuspension of old sediment or organic matter and adsorption of DOC onto POC susp (Druffel et al., 1992(Druffel et al., , 1996(Druffel et al., , 2003Bauer and Druffel, 1998;Bauer et al., 2002). Flocculating particles due to heterotrophic activity could also play a role in depleting PO 14 C susp (Druffel et al., 1992). Bauer et al. (2002) suggested that deep shelf PO 14 C from the MAB could be caused by natural hydrocarbon seepage; however, at the time of collection there was no evidence of such seepage, leading Bauer et al. (2002) to conclude that the depletion was due most likely to resuspended sediment. The similar correlation of δ 13 C and Δ 14 C observed between particulates from the MAB (Bauer et al., 2002) and the Desoto Canyon of the Gulf of Mexico (Figure 1) (Cherrier et al., 2014) suggests similar sources of carbon. In 2014, Skarke et al. (2014) reported the discovery of a major hydrocarbon seep field in the same area of the MAB that Bauer et al. (2002) had  Druffel et al., 1996) showed no trend, indicating a single source, surface primary production, to the POC susp . The covariation in δ 13 C and Δ 14 C for POC susp from the Mid-Atlantic Bight (MAB, open squares;Bauer et al., 2001Bauer et al., , 2002 and Desoto Canyon in the Gulf of Mexico (solid triangles; Cherrier et al., 2014) indicates the incorporation of another carbon source. Data for DIC from MAB are from Bauer et al. (2001) sampled. The hydrocarbons from this seep field could well be the cause of the correlation between the depleted δ 13 C and Δ 14 C of POC from the MAB (Figure 1). In April of 2010, the DWH Blowout released 717-789 million liters (4.5-4.9 million barrels) of oil and 500,000 t of gaseous hydrocarbons into the northern central Gulf of Mexico (Lehr et al., 2010;Joye et al., 2011). An estimated 30% of the released hydrocarbons formed a deep-water hydrocarbon plume between 1000-m and 1200-m depths (Valentine et al., 2010;Ryerson et al., 2012). The bulk of the gaseous hydrocarbons were primarily methane (Joye et al., 2011), but less than 0.01% of the gases reached the surface (Kessler et al., 2011;Yvon-Lewis et al., 2011). Crespo-Medina et al. (2014) measured methane oxidation rates in the water column following the DWH event. At the depth of the deep-water hydrocarbon plume, concentrations of methane and the gene methane monooxygenase (pmoA) were elevated, as were methane-oxidation rates. Cherrier et al. (2014) presented evidence that this DWH-derived CH 4 was found in the POC susp of the Gulf in 2011-2012 (Figure 1). Assimilation of methane by methanotrophs has been found to be very efficient in other systems, e.g., converting 63-85% of methane into biomass at landfill sites (Börjesson et al., 1998(Börjesson et al., , 2001. Du and Kessler (2012) estimated, using theoretical calculations of oxygen usage from the dissolved oxygen anomaly present in the deepwater hydrocarbon plume, that hydrocarbon degradation generated 0.36 ± 0.11 mg biomass per mg hydrocarbon. They also estimated that 0.10 ± 0.11 Tg of hydrocarbons, primarily methane, were converted into microbial biomass within the deep-water hydrocarbon plume. This biomass, including after cell senescence and viral lysis, would be an input to the POC susp pool.
Methane can also be generated microbially in the oceanic water column under aerobic conditions (Karl et al., 2008). This process occurs in the Gulf of Mexico (Rakowski et al., 2015), as evidenced by CH 4 concentrations and relative microbial abundances co-varying significantly from the seafloor to the euphotic zone. Thus, our characterization of the sources contributing to POC susp included characterizing the isotopic composition of biogenically produced methane.
Several studies following the DWH event analyzed POC and plankton in the GOM, finding depleted δ 13 C and Δ 14 C signatures (Graham et al., 2010;Chanton et al., 2012;Cherrier et al., 2014). Chanton et al. (2012) and Cherrier et al. (2014) found a linear relationship between δ 13 C and Δ 14 C signatures, from modern photosynthetic production to a hydrocarbon endmember, with both the plankton and POC falling along the line, indicating the incorporation of material originally sourced from hydrocarbons, as well as the movement of this material up the food web (Wilson et al., 2016).
The purpose of this study was to characterize δ 13 C and Δ 14 C signatures of POC susp in the GOM following the oil spill and as the system recovered from it, determining a new post-spill baseline for δ 13 C and Δ 14 C signatures of POC susp in the GOM in the process. (For parallel work on POC sink , see Chanton et al., 2018.) In the Atlantic and Pacific Ocean basins, the baseline derives from a single dominant source, modern photosynthetic production. No other carbon source contributes more depleted 13 C or 14 C to the pool of POC susp , so that co-variation between the isotopes (as in Figure 1) is not observed. In the GOM, however, the presence of two distinct carbon sources, one associated at times with anthropogenic activity, drives the depletion of both δ 13 C and Δ 14 C of POC, resulting in the co-variations depicted in Figure 1. The continuous input of hydrocarbon-derived material, petrocarbon, from natural seeps could also cause baseline signatures of the GOM to be more depleted than those from the Atlantic or Pacific oceans where input of hydrocarbon-derived material is quantitatively unimportant.
In this study we pursued three goals. First, we tested the hypothesis that, at sites with one-time (DWH) inputs of petrocarbon derived from methane and oil, initial depletion of the carbon isotopic signatures of POC susp would be followed by recovery, shifting the depleted values observed by Cherrier et al. (2014) towards more enriched baseline-like signatures. Assessing baseline signatures of POC susp in the Gulf prior to the DWH spill in 2010 has been difficult, as no PO 14 C data were collected in the GOM prior to that time. Second, we addressed the question, to what extent are the carbon isotopic values of POC susp in the GOM affected by the seep sites that provide a continuous source of hydrocarbon-derived material to the Gulf as opposed to the one-time input from the DWH? Third, we aimed to determine the relative importance of modern surface marine production and riverine inputs to the different carbon sources in the Gulf, as reflected in POC susp . In this study, we analyzed the stable carbon and radiocarbon isotopes of POC susp collected from across the northern GOM to determine the carbon sources to these particulates. For sources we considered surface marine primary production, riverine input, sediment, two sources of biodegraded methane, and biodegraded oil.

Material and Methods
Sampling and sample preparation POC susp samples were collected during fourteen cruises over six years from 2010 to 2017 (except 2011), from a total of 43 separate sites across the northern Gulf of Mexico (Figure 2). At the time of collection, sites were classified as seep or non-seep, with seep samples collected directly over areas of seafloor seepage, while non-seep samples (designated north central Gulf, NCG) were collected in areas not directly influenced by natural seepage. These determinations were based on map data from MacDonald et al. (2015) and shipboard acoustics used to detect hard bottoms or bubbly streams indicative of a seep site. Over the course of sampling we collected particles from 13 seep and 30 NCG sites.
Water column samples were collected by CTD-Rosette, filtering 1-20 L of water through pre-combusted 47-µm 0.7 GF/F borosilicate filters in plastic housings under gentle pressure filtration (5-10 psi). Filters were stored frozen in combusted aluminum foil and brought back to the lab for acidification in a filter housing unit with dilute (1N) HCl (Fernández-Carrera et al. 2016). Stable carbon isotope ratios of POC susp were analyzed on subsections of the filters using a Carlo-Erba elemental analyzer connected to a Finnegan MAT delta Plus XP Stable Isotope Ratio Mass Spectrometer (EA-IRMS) at the National High Magnetic Field Laboratory. The results were converted into δ 13 C with respect to VPDB (Vienna Pee Dee Belemnite). Samples analyzed after 2014 had a small split, about 10% of the total sample, of CO 2 removed prior to graphitization to be analyzed for δ 13 C. After δ 13 C analysis, the remaining filter was combusted, and the resultant CO 2 was purified cryogenically using the methods of Peterson et al. (1994). The purified CO 2 was flame-sealed in a 6-mm ampule and sent to Woods Hole National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS), University of Georgia Center for Applied Isotope Studies (UGA) or Lawrence Livermore National Laboratory Center for Acceleration Mass Spectrometry (LLNL CAMS) for analysis of natural abundance of radiocarbon. The radiocarbon signatures are reported in the Δ 14 C notation as described in Stuiver and Pollach (1977). The blank correction as described in Fernández-Carrera et al. (2016) was applied to both δ 13 C and Δ 14 C values. Forty coal samples, representing fossil 14 C dead carbon, were run to access our procedural blank over the course of this study. The average Δ 14 C value was -995 ± 7‰. We also ran 25 azalea leaf standards collected in Tallahassee, Florida, in 2013. The average Δ 14 C value was 31 ± 8‰. There was no variation between AMS labs in these samples or the coal blanks.
To estimate the DOC blank adsorbed onto the filters, we attached a second GF/F filter below the filter collecting the POC susp . The top filter would collect the POC, leaving the second filter to adsorb only DOC (if that were occurring). From this process, we estimated the DOC blank on the filters to be 0.12 µmoles CO 2 , representing 0.3 to 2% of the amount of carbon on the filters.
For water-column CH 4 and its δ 13 C isotopic composition, water samples were collected by CTD-Rosette and dispersed to glass bottles in June and July of 2013 at two sites in the northern GOM at 28.669°N; 88.3584°W, and at 28.32554°N and 88.3865°W (Figure 2). Methane concentrations were determined by the methods detailed in Magen et al. (2014). The stable isotopic composition of water-column methane was determined from 4-L bottles preserved with KOH as described in Magen et al. (2014). A headspace was introduced into the bottles and flushed into cryogenic trapping system, cryo-focused and run on a Thermo Finnegan Delta V Isotope Ratio Mass Spectrometer at Florida State University. The results were converted into δ 13 C with respect to VPDB (Vienna Pee Dee Belemnite).

Temporal trends in non-seep site data
The weighted average of the NCG POC susp per collection year for both Δ 14 C and δ 13 C was calculated to estimate the inventory of carbon and their isotopic signatures in the water column. The weighted average of radiocarbon was calculated by multiplying the total µmoles CO 2 sent for radiocarbon analysis for each year by the individual sample size of µmoles CO 2 . This proportion was then multiplied by the Δ 14 C for each sample and summed for the overall weighted average for that year. For the δ 13 C samples, the value for mg C per subsample used for δ 13 C analysis was calculated from the regression of the %C and δ 13 C of known standards. This value was extrapolated to the whole filter, assuming the sample was spread evenly. The value for total mg C for each sample was converted to µmoles and then summed following the radiocarbon method above.

Mixing models
A two-endmember mixing model based on Δ 14 C was used to estimate the percent carbon incorporated from photosynthesis and from all petrocarbon sources, combining methane and oil. The following equation was used to determine the percent carbon from modern surface production, with the denominator equaling the total range of radiocarbon from 39‰ (for marine production; Chanton et al., 2018) to -1000‰: The percent from hydrocarbons is 100 -% modern . We completed a sensitivity test for the two-endmember model by adjusting the total range of Δ 14 C by the standard deviation of the modern endmember. The Δ 14 C of the modern endmember was the average GOM plankton value from Chanton et al. (2018), 39 ± 26‰ (n = 79).

Statistics
Statistics were performed using R 3.1.3 R Core Team (2015). The data were not normally distributed, as indicated by the results of Levene's test. Therefore, the non-parametric Mann-Whitney U Test was used to compare the different groups for both δ 13 C and Δ 14 C. We continued the designation of seep versus NCG (non-seep, north-central Gulf) and divided our samples further into two depth categories (euphotic <300 m and deep water >300 m). The four categories we examined were as follows: seep euphotic, seep deep, NCG euphotic, and NCG deep. We used the adjusted p-value of 0.008 to indicate significance, accounting for the multiple comparisons (α = 0.05, 6 comparisons). From the z value we calculated the effect size: where r is the effect size, z is the z score, and N is the sample size.
The δ 13 C and Δ 14 C signatures for all POC susp samples, across all collection depths and sampling years, were highly variable, as seen in Figure 3 where the δ 13 C and Δ 14 C signatures of NCG POC susp are color-coded by year. Suspended particles from seep sites were not colorcoded by year, as the majority of the samples were collected in 2013. We observed two trends of δ 13 C and Δ 14 C co-variation in the POC susp data (Figure 4). One trend, the lower limb, indicated co-variation based on blending of modern surface production and petrocarbon input, observed for both NCG and seep sites. The upper limb, observed primarily for NCG sites, reflected the addition of another carbon source to modern production, with depleted δ 13 C but more enriched Δ 14 C signatures, possibly associated with the biodegradation of biogenic methane produced in the water column (Rakowski et al., 2015). In this study, we used two different mixing models to characterize both the co-variation based on petrocarbon input and the variation based on all of the sources that could contribute to GOM POC susp .
Study-wide concentrations of POC susp followed an expected gradient with highest concentrations at the surface decreasing with depth ( Figure 5). Concentrations in the euphotic zone (<300 m) ranged from 0.32 to 62.5 µM (mean ± S.D.: 4.71 ± 9.38, n = 78). The four highest concentrations at the surface came from non-seep sites that were heavily influenced by  riverine input. Concentrations of POC susp in deep water (>300 m) ranged from 0.16 to 12.02 µM C (mean ± S.D.: 1.44 ± 2, n = 72).

Temporal trends in carbon isotopes in POC susp
We tested the hypothesis that δ 13 C and Δ 14 C of POC susp in the north central Gulf at non-seep sites would vary temporally, becoming increasingly enriched in heavy isotopes following their depletion associated with the 2010 injection of fossil hydrocarbons into the water column from the DWH oil spill (Cherrier et al., 2014;Fernandez et al., 2016;Weber et al., 2016). We calculated the weighted averages of the non-seep NCG particles collected each year to create an inventory that accounts for the quantity of organic carbon contributing to the δ 13 C and Δ 14 C signatures ( Table 2) Figure 6D).

Biogenic methane
Dissolved CH 4 concentrations in the water column varied from 2.6 to 11.6 nM, while the isotopic composition of methane varied from -37 to -52‰. A subsurface maximum in methane concentration was observed in both profiles within the euphotic zone at 60-75 m depth (Figure 7). The average δ 13 C value, weighting the two profiles equally, was -41.4 ± 1.0‰ (n = 19). For the mixing model, we assumed that this methane was produced from modern photosynthetic carbon, not derived from nearby seeps, and had a Δ 14 C value of 39 ± 26‰ (n = 79). The δ 13 C value we observed was similar to those measured in the Atlantic and Pacific oceans which varied between -43 and -45‰ (Holmes et al., 2000). Karl et al. (2008) have suggested that oceanic water column CH 4 is produced aerobically as a by-product of methylphosphonate decomposition in phosphate-stressed waters, supporting our assumption of a modern 14 C value for this methane. Rakowski et al. (2015) observed depletion of phosphate in the euphotic zone at the methane maximum, where we similarly observed it. The 13 C value is consistent with production from a methylated substrate in limited supply that is consumed quantitatively (Kelley et al., 2012;Tazaz et al., 2013).

Mixing models
The two-endmember mixing model indicated that the bulk of the carbon in POC susp from the NCG euphotic (<300 m), seep euphotic and NCG deep (>300 m) sites was derived from modern surface production ( Table 3). In contrast to seep euphotic, seep deep POC susp had incorporated the most petrocarbon, averaging 57% ± 13 (n = 9), while NCG deep POC susp averaged 29% ± 16 (n = 39) petrocarbon (Table 3). Our sensitivity test showed the greatest variation in the euphotic POC susp , where Δ 14 C was close to the modern endmember. The two-endmember model estimations of the %modern for the euphotic test samples were within 2-3% of the model values, and the %petrocarbon estimations also varied by 2-3%. The deep-water source estimations varied by 0-1% for both %modern and %petrocarbon.
The MixSIAR model (Table 3) suggested that POC susp in the north central GOM is heavily derived from riverine inputs (up to 46%), followed by modern surface production (up to 45%), and, at seep deep sites, by oil (up to 46%). All deep-water POC susp had higher contributions from hydrocarbons than euphotic POC susp . Euphotic POC susp at seep sites also had higher contributions from oil-derived carbon than at NCG euphotic sites; i.e., 8.2 ± 3.6% (n = 14) compared to 2.0 ± 1.2% (n = 55). Sediment, fossil methane, and biogenic methane contributed very little to the organic carbon in POC susp , with high standard deviations of their means.

Broader context
To characterize POC susp in the broader context of the GOM, we plotted the δ 13 C and Δ 14 C signatures for other carbon reservoirs in the GOM, including: POC sink , nonseep sediment, seep sediment, and DIC (Figure 8). We also included signatures (as provided in Methods) for the different potential carbon sources to these pools, including modern surface production, riverine input, sediment, biogenic methane, DWH methane, and oil (Figure 8).

Statistics
The data were non-normally distributed; therefore, the non-parametric Mann-Whitney U tests were used to analyze the variation of δ 13 C and Δ 14 C signatures of POC susp , from 2010 to 2017. We compared the importance of depth in the water column (euphotic <300 m or deep >300 m) and site type (NCG or seep) in determining the isotopic signatures of POC susp (Tables 4 and 5) using the adjusted p-value of 0.008 to define significance. There were significant differences in δ 13 C signatures when comparing between depths, regardless of site classification.

Discussion
The first goal of this study was to characterize any temporal trends in the δ 13 C and Δ 14 C signatures of suspended POC in the northern Gulf of Mexico following the DWH oil spill in 2010. Isotopically depleted values for POC susp indicating a fossil petrocarbon source were observed following the spill (Figure 6), particularly in the 14 C content of suspended particles below the euphotic zone at NCG sites. The influence of apparent DWH-derived material was greatest in 2011 and 2012, with recovery beginning thereafter and proceeding until 2014 when the isotopic composition of POC susp reached an asymptotic value (Figure 6).  Chanton et al. (2018) found that POC sink recovered in 1-3 years depending on the tracer that was evaluated. δ 34 S and PAH indicated an approximate 2-year recovery time, while Δ 14 C indicated a recovery time of ~3 years. These recovery periods are on a similar time scale to our estimate of a 4-year recovery period in the Δ 14 C of POC susp in deep water at NCG sites (Figure 6).
A second goal was to determine the extent of fossil carbon influence on suspended particles. We found a wide range of natural variability in both δ 13 C and Δ 14 C signatures of POC susp across the northern GOM, from seep and non-seep sites and a range of depths, from surface to 1900 m (Figure 3). Compared to suspended particles in the Sargasso Sea and Pacific Station M, the GOM exhibits more variability and greater depletion in δ 13 C and Δ 14 C   (Figure 1; Druffel et al., 1992Druffel et al., , 1996. The primary carbon source to the Sargasso and Pacific particulates is modern photosynthetic production, which does not create a co-variation of δ 13 C with Δ 14 C. The depletion in Δ 14 C in the Sargasso and mid-Pacific was not observed to be greater than -100‰, even to depths of 4000 m. The δ 13 C value of the Pacific and Atlantic suspended particles was generally -20 to -22‰ without depth variation. The co-variation of δ 13 C and Δ 14 C observed in the GOM and the Mid-Atlantic Bight (as presented in the Introduction) are due to the incorporation of a second source that is depleted in both 14 C and 13 C, consistent with petrocarbonderived material. In the Southern Gulf of Mexico from 20° to 22°N, Gonzalez-Ocampo et al. (2007) reported POC susp depth trends in δ 13 C, with 13 C depletion at depth reaching values as low as -23.7 ± 0.5‰. Values in surface water were -22.5 ± 0.5‰.
We admit some reservations about the results of the MixSIAR mixing model, as two of our sources, riverine input and sedimentary organic carbon had some surprising results. The MixSIAR model was used to constrain six carbon sources with two isotopic tracers. The riverine input (34-46%) was estimated to be greater than the input from surface production (11.6-45.1%) for all sites except NCG euphotic (riverine: 41%; surface production: 45%). For the workings of the model, the riverine endmember was located isotopically in the middle of the bulk of the POC susp values, rather than along the boundary of our data (Figure 4). This location could cause the model to estimate a higher percent contribution to POC susp because of the isotopic similarities between the POC susp and riverine input. The data might well represent mixing between modern carbon and a more depleted source. On the other hand, the apparent high riverine contribution may be due to the differences in lability between the riverine and modern sources. Riverine carbon is less labile than fresher photosynthetic production, which causes it to cycle more slowly than surface production (Wang et al., 2004). The salinity at the stations where we collected POC susp varied (Table S1), but most sites did not indicate major mixing with freshwater sources. However, Wang et al. (2004) found that δ 13 C and C:N ratios of POC susp from the Mississippi and northern GOM exhibited non-conservative behavior when mixing with higher salinity waters. A decoupling occurs between the POC susp and the freshwater input which allows the 2.30 Tg POC yr -1 exported from the Mississippi (Cai et al., 2015) to mix with GOM POC susp and accumulate over the POC susp residence time of 5-10 years, increasing the contribution from riverine POC susp .
Whereas the riverine contribution to POC susp was higher than expected, the sedimentary contribution was low and consistent throughout the water column. Chanton et al. (2018) found that sinking POC, collected from traps 30 m above the seafloor, did not carry a strong signal from resuspended sediment, but POC susp integrates over longer time scales. Diercks et al. (2018) detected both small-scale and hurricane-sized resuspension events, which could play a role in the transport of sedimented petrocarbon. The number and overall scale of resuspension events in the GOM is unknown, but they potentially introduce more than 4-6% into the POC susp pool, especially near the seafloor. The sedimentary organic carbon endmember is similar in isotope space to riverine input (Figure 4), making the two sources difficult to separate. Employing sulfur isotopes and lithogenic silica content might better separate sedimentary and riverine sources. Unlike the MixSIAR model, the two-endmember model is simpler and better constrained. This model indicated the local influence of seeps, particularly on deep-water POC susp , and the importance of modern surface production on POC susp away from seeps (Table 3). Nonetheless, petrocarbon still contributed about 30% of POC susp in the deep GOM, even away from seep sites, a phenomenon not observed in the Atlantic or Pacific. Even though the two-endmember model does not capture the full complexity of all of the potential sources to POC susp in the GOM, the results of the MixSIAR model, using only two isotopic measurements, can only be fully interpreted with reservations, for the aforementioned reasons. Therefore, we have greater confidence in the two-endmember approach.
We estimated the new, post-spill baseline signatures for NCG (non-seep) POC susp in the GOM to be δ 13 C = -24.1 ± 0.4‰ (n = 15) and Δ 14 C = -17 ± 36‰ (n = 15) for the euphotic zone and δ 13 C = -27.5 ± 1.1‰ (n = 12) and Δ 14 C = -164 ± 19‰ (n = 7) for deep-water suspended particles. These values are the means of the POC susp weighted averages from the NCG for the last three sampling years (2015-2017; Figure 6). Following the recovery of deepwater Δ 14 C signatures in 2014, we suggest that POC susp reached a post-blowout baseline, as both the δ 13 C and Δ 14 C signatures had stabilized by these years. In comparison to baselines from the Sargasso Sea and Pacific Ocean, the baseline for POC susp from the GOM was more depleted for δ 13 C and Δ 14 C. We estimated the δ 13 C and Δ 14 C baselines for POC susp collected in the Sargasso and Pacific (Druffel et al., 1992(Druffel et al., , 1996(Druffel et al., , 2003 for our depth zones to be: euphotic δ 13 C = -22.1‰ (n = 41) and Δ 14 C = 73 (n = 42); and deep POC susp δ 13 C = -21.2‰ (n = 82) and Δ 14 C = 16‰ (n = 83). The differences in δ 13 C between the GOM and the Sargasso and Pacific are due to differences in the increased relative importance of hydrocarbon and terrestrially sourced material to the GOM, while the Δ 14 C differences are caused by both collection years and the relative importance of differing carbon sources. The gap of 22-32 years between the collection of the Sargasso and Pacific samples and our samples results in the older samples being more enriched in 14 C. This effect is due to the atmospheric nuclear testing that caused radiocarbon values to spike and mix from the atmosphere into the biosphere and hydrosphere, including the ocean. The mixing process caused a lag between atmospheric CO 2 radiocarbon signatures and DIC in the ocean (Levin and Hesshaimer, 2000;McNichol and Aluwihare, 2007).
Unlike POC susp from the Sargasso Sea and Pacific Ocean, depth played a significant role in the variation of δ 13 C of POC susp in the GOM over the period of 2010-2017. The significant differences observed between euphotic (<300 m) and deep (>300 m) reflect the relative importance of the two sources, with petrocarbon-derived material increasing in importance below 300 m (Tables 2 and  4). Suspended particles in deep water were depleted due to the hydrocarbon input potentially from seep sites and/or the DWH blowout, while POC susp from the same depth (whether euphotic or deep) had similar δ 13 C, regardless of site type (seep or NCG).
We also found significant differences between the Δ 14 C of POC susp in euphotic and deep POC susp for both site types (seep or NCG) ( Table 5). This finding suggests that suspended particles in the GOM, including at non-seep sites, are more depleted in 14 C in deeper waters than suspended particles in other oceans, probably due to the natural hydrocarbon seepage in the GOM, although completely ruling out the lingering petrocarbon from the oil spill is difficult at this point. The significant differences between the Δ 14 C of deep-water POC susp from NCG and seep areas were not observed for the δ 13 C results, as δ 13 C is less sensitive to variations in the input terms (Bosman et al., 2016). The more depleted Δ 14 C of POC susp found at seep sites suggests that the presence of natural hydrocarbon seepage significantly affects the Δ 14 C of suspended particles in the Gulf.

Conclusions
Based on our assessment of the δ 13 C and Δ 14 C signatures of POC susp , petrocarbon is an important carbon source to suspended particles in the GOM, in contrast to the situation in the Atlantic and Pacific oceans. In the north central Gulf during our study period, POC susp was isotopically depleted relative to POC sink or sediment (Figure 8). DIC fixed by marine primary production is the primary source of carbon to POC sink and to the non-seep sediment. We found deep POC susp from seep sites to be composed of about 45% oil-derived petrocarbon, while deep-water POC susp from the NCG sites may contain as much as 15% oil-sourced petrocarbon and 3.5% methane-sourced petrocarbon. In the Gulf of Mexico, there are at least three clear and separate carbon sources (Figure 8): one driven by surface primary production, observed in POC sink and non-seep sediments; the second attributed to the microbial cycling of methane and oil, observed to a greater extent in POC susp and seep sediments; and the third, a riverine contribution, pervasive but more challenging to quantify unambiguously.

Data Accessibility Statement
Data deposition: Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org, 10.7266/N7FX77C7.

Supplemental file
The supplemental file for this article can be found as follows: • Table S1. POC susp sample collection and isotope data. DOI: https://doi.org/10.1525/elementa.389.s1