Rising permafrost temperature (Romanovsky et al., 2010), declines in sea ice extent (Overeem et al., 2011), and longer ice-free seasons (Stammerjohn et al., 2012) are increasing the vulnerability of Arctic coasts to erosion (Manson and Solomon, 2007). Arctic coastlines, which make up roughly one third of the global coastline extent, are retreating at an average rate of 0.5 m year⁻¹ (Lantuit et al., 2012). The long-term mean annual erosion rate (1940–2000s) of Alaska’s 1,957 km of Beaufort Sea coastline is 1.7 m year⁻¹, but there is high spatial and temporal variability in erosion rates (Gibbs and Richmond, 2015). Not only is this coast retreating on average, but erosion rates along the Alaska Beaufort Sea coast are also increasing, from 0.6 m year⁻¹ circa 1950–1980 to 1.2 m year⁻¹ circa 1980–2000 (Ping et al., 2011). Some sections of the Alaska Beaufort Sea coast retreat more than 22 m in a single year, which is on par with the fastest eroding coasts worldwide (Gibbs and Richmond 2015; Jones et al., 2009; Jones et al, 2018). Rapid erosion along northern Arctic coastlines threatens infrastructure and cultural heritage sites of local communities (Jones et al., 2008). Erosion also delivers sediment, organic matter, and inorganic nutrients to the Arctic Ocean (Ping et al., 2011; Lantuit et al., 2012).

Soils in northern Alaska contain ∼40–150 kg total organic carbon (TOC) m⁻² in the top 3 m (Strauss et al., 2013; Hugelius et al., 2014; Schuur et al., 2015; Fuchs et al., 2019). Most of this TOC is sequestered in permafrost; however, coastal erosion can rapidly liberate large quantities of this frozen organic matter into the coastal ocean (Vonk et al., 2012; Vonk and Gustafsson, 2013; Fritz et al., 2017; Stein and MacDonald, 2004). An estimated 4.9–14.0 Tg of organic carbon is released to the Arctic Ocean each year from coastal erosion (Wegner et al., 2015), similar in magnitude to river-borne particulate organic carbon (POC) export to the Arctic Ocean (5.8 Tg; McClelland et al., 2016). Eroded material may be deposited as nearshore marine sediments, resuspended in the water column as POC, or produce dissolved organic carbon (DOC) through physical leaching processes (Dou et al., 2008; Vonk et al., 2012; Fritz et al., 2017). Previous work has shown that organic matter in coastal permafrost along Herschel Island, Canada is rapidly decomposed upon entering seawater, thereby producing carbon dioxide via microbial metabolism (Tanski et al., 2019). In addition to heterotrophic bacteria, higher organisms in nearshore marine food webs also incorporate terrestrial organic matter from along the Beaufort Sea coast (Dunton et al., 2012; Bell et al., 2016; Harris et al., 2018).

TOC storage varies widely along the northern coast of Alaska due to differences in geomorphology and surficial depositional history, making it difficult to constrain estimates of erosional organic matter fluxes to the ocean (Jorgenson and Brown, 2005; Dou et al., 2010; Ping et al., 2011). For instance, Ping et al. (2011) estimates that TOC storage on the Alaska Beaufort Sea coast is highest for exposed bluffs and bays (92–99 kg TOC m⁻²) and lowest in tapped lake basins and deltas (41–46 kg TOC m⁻²). When paired with measurements of coastline loss, TOC storage estimates translate into erosional fluxes of 153–180 Gg TOC to the Alaska Beaufort Sea annually (Jorgenson and Brown, 2005; Ping et al., 2011).

In this study, we estimate organic matter and inorganic nutrient fluxes from a 9 km length of coastline near Drew Point, located approximately 100 km east of Utqiaġvik (formerly known as Barrow), AK. The exposed coast between Drew Point and Cape Halkett has the highest rates of erosion along the northern coast of Alaska (Gibbs and Richmond 2015). Jones et al. (2018) reports that erosion along this 9 km segment has more than doubled since the mid-20th century, with recent (2007–2016) annual erosion rates averaging 17.2 m yr⁻¹. This region of Alaska’s North Slope is underlain by continuous permafrost, where the upper several meters of permafrost are characterized by high ground-ice content (up to 90% in some locations), with an abundance of wedge ice (Kanevskiy et al., 2013). Throughout the Holocene, widespread thermokarst lake formation and drainage in this region left roughly 84% of the landscape covered by lakes or drained thermokarst lake basins (DTLBs) (Hinkel et al., 2005; Jones and Arp, 2015). Below reworked lacustrine sediments, permafrost material is composed of marine sediments that were deposited during late-Pleistocene marine transgressions (Black, 1983; Rawlinson, 1993; Brigham-Grette and Hopkins, 1995). The coastline consists of north-facing, exposed bluffs that reach elevations of 1.6–7.1 m above sea level (Jones et al., 2018). Along these bluffs, the primary mechanism for erosion occurs via thermo-mechanical development of niches from wave-action during storms that cause large blocks of permafrost to collapse into the ocean (Jones et al., 2009).

Estimates of organic matter stocks in eroding soils along the Alaska Beaufort Sea coast are currently based on only a few studies. We constructed geochemical profiles extending from the tundra surface to below sea level using coastal permafrost cores (4.5–7.5 m) to better constrain organic matter and nutrient stock estimates along one of Alaska’s most rapidly eroding stretches of shoreline. This is one of the first studies to compare stocks of bulk organic matter with porewater dissolved organic matter in eroding permafrost. Additionally, ¹⁴C data from core sediments provide new insight on landscape geomorphology and carbon accumulation rates over time. We used geospatial imagery and LiDAR data to calculate volumetric erosion rates over three time intervals from 1955 through 2007 and over annual time steps from 2008 through 2017 for a 9 km length of coastline. These erosion rates were combined with geochemical data to estimate TOC and other constituent fluxes to the Beaufort Sea from this study coastline. While the results of this study are particularly relevant to the Drew Point area, they also serve as a benchmark for improving regional-scale estimates of erosional organic matter inputs to Arctic coastal waters.

Our geochemical analyses show that two distinct permafrost layers are eroding into the ocean near Drew Point, AK: 1) organic-rich Holocene-era terrestrial soils and lacustrine sediments and 2) late-Pleistocene marine sediments. The upper layer, found above 3 m elevation, contains TOC with ¹⁴C ages as old as ∼12 kyr BP. The δ¹³C-TOC values in this layer (averaging −27.9‰) are consistent with contributions from C3-dominated tundra vegetation in the Arctic (Stein and Macdonald 2004). Higher C:N ratios above 3 m elevation (Figure 5) are also indicative of vascular plant contributions. Studies have shown that thaw lake development in this region was initiated by the Holocene Thermal Maximum approximately 9–11 kyr BP (Jones and Yu 2010; Kaufman et al., 2004), which aligns with the rapid accumulation of organic-rich material in our cores since 12 kyr BP. Below 3 m elevation, the shift to higher δ¹³C-TOC values (Figure 5) reflects an increase in marine-derived organic matter. The lower TOC content (<2%) and C:N (∼8–14) of organic matter in the lower layer are also indicative of its marine origin, having similar values to organic matter in nearshore sediments of the Beaufort Sea (Stein and Macdonald, 2004; Couture et al., 2018; Goni et al., 2000). Other studies along Beaufort Sea coastal bluffs report permafrost containing marine microfauna and refer to this silty, unconsolidated sediment as the Flaxman Member of the Gubik Formation (Black, 1983; Carter et al., 1988; Rawlinson, 1993). Thermoluminescence dates on Flaxman Member sediments suggest that this material accumulated during the Simpsonian marine transgression approximately 70–80 kyr BP (Dinter et al., 1990; Rawlinson, 1993; Brigham-Grette and Hopkins, 1995). Another study inland from Drew Point also provides evidence for a late Pleistocene marine transgression occurring between 113 and 71 kyr BP (Farquharson et al., 2018). The relatively young (∼40 kyr BP) radiocarbon ages of bulk OM that we measured below 2 m elevation suggest that these marine sediments have been reworked and subject to homogenization across depths. Our estimated net accumulation rates were comparatively low between 12 and 38 kyr BP, corresponding to colder, drier conditions in the late Pleistocene before rapid development of peatlands (MacDonald et al., 2006; Jones and Yu, 2010). A study by Meyer et al. (2010) showed similar sedimentology near Utqiaġvik, AK, where high-salinity sediments at depth were assigned a¹⁴C–TOC age of 39.5 kyr BP, and there was a geochronological gap between 21.7 and 12.4 kyr BP that suggests there was little deposition or reworking during this time.

The unfrozen, brine-rich permafrost (cryopeg) that we encountered near sea level is consistent with findings by Brown (1969) and Meyer et al. (2010) near Utqiaġvik, AK. Proposed mechanisms for brine formation include 1) rejection and concentration of salts in porewaters of soils and lacustrine sediments during permafrost aggradation and 2) residual seawater from relict marine sediments (Brown, 1969; Cascoyne, 2000). Our results showing a decrease in strontium isotope ratios in porewaters with a salinity above ∼10 (Figure 7) suggest that both mechanisms are likely involved. In porewater samples with salinities greater than 10, Sr isotope ratios of 0.7094–0.7095 are similar to values of ∼0.0792 found in modern seawater (Krabbenhoft et al., 2010; Peucker-Ehrenbrink et al., 2010). In porewater samples with salinities less than 10, on the other hand, Sr isotope ratios transition from values close to those expected for seawater to values expected for North American fresh waters (>0.71; Krabbenhoft et al., 2010; Peucker-Ehrenbrink et al., 2010). This points to a mixture of marine and freshwater-derived ions, with the marine source dominating at porewater salinities above 10. Regardless of the mechanism of formation, the existence of a cryopeg along the base of bluffs has implications for coastal erosion, as unfrozen and unconsolidated cryotic soils may be more susceptible to erosion by wave action (Jones et al., 2018).

Most of the landscape near Drew Point has been reworked by thermokarst lake formation and drainage, where ice-rich permafrost degradation and ground ice melt lead to lake formation, followed by drainage and subsequent soil organic matter accumulation and ice-wedge growth (Bockheim et al., 2004; Jorgenson and Shur, 2007; Jones et al., 2012). Thermokarst lakes have been shown to drain quickly, releasing large quantities of water and dissolved organic matter (Jones and Arp, 2015). Post drainage, a study from the Seward Peninsula, Alaska showed that peat accumulates rapidly as productive species colonize the newly exposed surface (Jones et al., 2012). Although peat accumulation slows over time as surface conditions change, TOC stocks tend to increase with lake basin age (Hinkel et al., 2003; Jones et al., 2012). Our cores span a gradient in landscape geomorphology, representing a site where lake formation and drainage did not occur during the Holocene (primary surface), an ancient lake basin that formed and drained about 5 kyr BP, and a young basin where two lakes formed and drained with the last draining occurring in the last 0.5 kyr BP (Hinkel et al., 2003; Jones et al., 2012). We do not explicitly report geomorphic terrain unit-specific carbon stocks in this study because more replication of primary surface, young DTLB, and ancient DTLB coring would be needed to do so. However, there is a general trend of increasing surface elevation with landscape age that translates into differences in carbon storage among geomorphic terrain units. Due to differences in surface elevation, our estimates of carbon storage from local sea level to the tundra surface range from 67.1 kg TOC m⁻² (3.0 m bluff typical of an intermediate age DTLB) to 106.3 kg TOC m⁻² (4.8 m bluff typical of primary surface). These storage estimates bracket values reported by Ping et al. (2011) for exposed bluffs with an average height of 3.2 ± 0.5 m along the Alaskan Beaufort Sea coast (92 ± 35 kg TOC m⁻²).

Our geochemical profiles demonstrate that sampling at elevations from near sea level to the active layer is necessary to account for variability in TOC stocks when calculating erosional fluxes. For example, if we calculate TOC stocks for a typical 4 m bluff using TOC and bulk density data from the top meter alone, we overestimate TOC stocks by ∼90% relative to those calculated using data from the full profile. We get closer if we use data from the top 3 m of soil, but still overestimate TOC stocks by ∼8%. Only when we fully account for down-core variations do the estimates completely converge. At least where marine transgressions come into play, accounting for the presence of relict marine sediments as well as organic-rich Holocene age soils and lacustrine sediments allows for more accurate quantification of TOC stocks.

The approximate doubling of TOC flux over the timeframe of our analysis (Figure 8) is associated with accelerating coastline erosion rates rather than a shift in landscape classification or surface elevation. The accelerated erosion rates translate into a higher estimated TOC flux (1,370 kg TOC m⁻¹ year⁻¹) than previously reported by Jorgenson and Brown (2005) or Ping et al. (2011) for the Drew Point coastline (818–848 kg TOC m⁻¹ year⁻¹). Ping et al. (2011) estimated that, on average, exposed bluffs contribute 163 kg TOC m⁻¹ year⁻¹ to the Beaufort Sea. Our estimates suggest that TOC fluxes from Drew Point are nearly an order of magnitude higher than the typical exposed bluffs along the Beaufort Sea coast. As mentioned previously, this is explained by higher erosion rates at Drew Point (17.2 m year⁻¹) compared to the average 1.8 m year⁻¹ (1950–2000) from exposed bluffs reported by Ping et al. (2011). For additional context, it is also noteworthy that annual TOC erosion from our 9 km study section at Drew Point (12,331 tons year⁻¹) is now on par with annual TOC inputs to the Beaufort Sea from the Kuparuk River (12,387 tons year⁻¹; McClelland et al., 2014). Arctic rivers are known to transport large quantities of organic matter and strongly influence the Arctic Ocean (Holmes et al., 2012; McClelland et al., 2016). These results are impressive considering the Kuparuk drains 8,107 km² of land extending from the Brooks mountain range to the Beaufort Sea coast and is the third largest river on the North Slope of Alaska (McClelland et al., 2014). The Kuparuk River empties into the Beaufort Sea near Deadhorse, approximately 200 km east of Drew Point.

Erosional TN fluxes follow similar trends to TOC, indicating that erosion also contributes a substantial amount of nitrogen to nearshore Arctic coastal waters. Over the past decade, we estimate that erosion from this coastline supplies 838 tons TN year⁻¹ to the ocean, similar to the TN flux (particulate organic nitrogen, dissolved organic nitrogen, ammonium, and nitrate) from the Kuparuk River (641 tons N year⁻¹; McClelland et al., 2014). While this eroding nitrogen is mostly in the particulate organic form, a fraction of it is likely remineralized to inorganic forms in the nearshore environment and supports marine primary production (Tank et al., 2012). It is also important to consider the timing of these nitrogen fluxes. Riverine fluxes peak during the spring snowmelt period, compared to erosion that peaks during the summer open-water season (Holmes et al., 2012; Jones et al., 2018). Therefore, coastal erosion likely supplies nitrogen to nearshore coastal environments during a time of nutrient depletion following spring phytoplankton blooms (Connelly et al., 2015; Kellogg et al., 2019).

Similar to down-core patterns in organic matter, the profiles of DOC, TDN and NO₃ ⁻ show differences in concentrations between Holocene age soils and lacustrine sediments and late-Pleistocene marine sediments. TDN (Supplementary Table S4) tracks DOC, and NO₃ ⁻ concentrations are <1% of TDN, indicating that the bulk of porewater nitrogen is in the organic form. Increasing DOC concentrations with depth from the tundra surface to the bottom of the Holocene age material, and again from ∼2 m elevation to the bottom of the core (Figure 6) could be attributed to leaching of particulate organic matter over time and the concentration of solutes during freeze-thaw cycles. NO₃ ⁻ concentrations decline with depth within terrestrial-derived soils to undetectable concentrations within relict marine sediments. Lower DOC:TDN molar ratios in porewater below 3 m elevation is also reflective of the transition to relict marine sediments. Despite the large volumes of pore and segregated ice that can be found in permafrost, porewater DOC and nutrient concentrations are a small fraction of the total mass fluxes of carbon and nitrogen. DOC and TDN accounted for less than 1% of the total mass fluxes of TOC and TN, respectively (Table 1). However, particulate organic matter eroding into the ocean may be quickly solubilized to dissolved organic forms (Dou et al., 2008) and remineralized to inorganic forms of nitrogen in the nearshore environment (Tank et al., 2012).

In this study, we evaluate the role of coastal erosion in mobilizing ancient stocks of organic carbon and nitrogen to the nearshore Beaufort Sea. Our estimates for carbon storage in the bluffs are comparable to previous studies, but our calculated annual fluxes of material are higher than previous estimates due to the rapid, increasing erosion rates in this region. This suggests that future studies should continue to monitor shoreline loss over time, particularly for sections of coast with high erosion rates or tall bluffs that contribute the greatest volumes of material. The unique, high-resolution depth profiles of TOC, ¹⁴C, and other geochemical constituents in this study highlight the need to consider landscape geomorphology and TOC content at depth to prevent overestimating carbon storage in deeper soil horizons. While permafrost contains large volumes of porewaters, we show that dissolved organic carbon and nitrogen are a small fraction of the total flux. Providing detailed estimates of eroding TOC and nutrient fluxes in a warming Arctic will inform our understanding of nearshore carbon and nutrient cycling. Arctic rivers are understood to deliver large quantities of terrestrially-derived organic matter to the ocean (Holmes et al., 2012; McClelland et al., 2016). This study emphasizes coastal erosion as another important driver of organic carbon and nitrogen fluxes to the ocean. Eroding coasts supply organic matter mostly in the particulate form; however, it is likely that dissolved organic matter is leached from eroding material in seawater and can be used by heterotrophic bacteria in the water column or marine sediments. Therefore, estimates of erosional organic carbon and nutrient fluxes, in combination with studies of organic matter lability, are necessary to assess CO₂ production as well as marine productivity in the Arctic Ocean.

The original contributions presented in the study are publicly available. This data can be found here: doi: 10.6073/pasta/cc4d53a91ed873765224fcb6d09f5eb7.

All authors were involved in developing the concept and design of this study. MK, GI, and BJ conducted field work. EB, CC, TL, AI, DB, and RC were involved in sample processing. EB, CC, TL, BR, and AI contributed to geochemical analyses. BJ provided geospatial analyses. EB, CC, BJ, and JM wrote the initial draft of the manuscript, and all authors contributed to subsequent revisions.

This project was primarily supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Additionally, the National Science Foundation provided support for BMJ (OPP-1806213, OISE-1927553), JWM (OPP-1656026), and MK (OPP-1820883). USGS personnel were supported by the USGS Coastal Hazards Program and the Alaska Coastal Processes and Hazards Project team. The Beaufort Lagoon Ecosystems LTER program (funded by OPP-1656026) also provided travel support for EMB’s field work, and provided assistance with data archiving.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy‘s National Nuclear Security Administration under contract DENA0003525. Honeywell International Inc. was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.