Assessing the impact of the global subsea telecommunications network on sedimentary organic carbon stocks

Jun 05, 2023

Nature Communications volume 14, Article number: 2080 (2023) Cite this article

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The sequestration of organic carbon in seafloor sediments plays a key role in regulating global climate; however, human activities can disturb previously-sequestered carbon stocks, potentially reducing the capacity of the ocean to store CO2. Recent studies revealed profound seafloor impacts and sedimentary carbon loss due to fishing and shipping, yet most other human activities in the ocean have been overlooked. Here, we present an assessment of organic carbon disturbance related to the globally-extensive subsea telecommunications cable network. Up to 2.82–11.26 Mt of organic carbon worldwide has been disturbed as a result of cable burial, in water depths of up to 2000 m. While orders of magnitude lower than that disturbed by bottom fishing, it is a non-trivial amount that is absent from global budgets. Future offshore developments that disturb the seafloor should consider the safeguarding of carbon stocks, across the full spectrum of Blue Economy industries.

Marine sediments are the largest store of organic carbon on Earth and this sequestration plays a key role in regulating global climate1,2,3,4. However, if previously-buried organic carbon stocks are disturbed and exhumed, this can lead to remineralization of carbon to CO2 (which could potentially increase ocean acidification), limiting the capacity of the ocean to store additional CO2, and potentially adding to the build-up of atmospheric CO23,4,5,6. Sedimentary carbon stocks can be episodically disturbed by natural events, such as floods, storms that resuspend shallow seafloor sediments, or large earthquake-triggered submarine landslides7,8,9,10,11. In addition to these natural events, human activities that impact the ocean floor (e.g. fishing, mining, oil and gas exploration, aggregate extraction, anchoring) are increasingly recognized as playing a significant role in the release of previously-buried organic carbon, with intensity and spatial extent growing by the increased use of marine resources and Blue Growth2,3,4,12,13,14,15,16. It is estimated that 1.3% of the global ocean-floor is trawled each year (∼5 × 106 km2), potentially releasing similar quantities of sedimentary organic carbon to agricultural tillage on land17. To what extent other human activities release previously-buried carbon remains unclear; largely due to lack of access to industry datasets that permit quantification of that disturbance. This limitation inhibits assessment of the impacts of the full extent of human activities on carbon burial efficiency worldwide. Here, we assess the potential impact of one of the most extensive infrastructure systems on our planet—the network of subsea telecommunications cables that span more than 1.8 million km across the global ocean (Fig. 1).

A Cable routes are colour coded (white to dark blue) according to water depth (m). Bathymetry derived from the GEBCO_2022 Grid, GEBCO Compilation Group (2022) GEBCO 2022 Grid. B Extent of cable routes in water depths of up to 2000 m illustrated as black lines, overlying global distribution map of sedimentary carbon stocks in the first meter below the seafloor from Atwood et al.2. Inset pie chart shows the relative length of all cable routes that cross different physiographic domains, based on global geomorphic mapping63. Country outlines sourced from Natural Earth free vector data.

More than 99% of all international digital data traffic is routed via >400 interconnected submarine cable systems (Fig. 1A), which underpin the Internet, enable remote working, financial transactions worth trillions of dollars per day, and connect remote island states to sustain their economic development18,19. These cables, which are either laid directly on the seafloor or buried and typically have a diameter equivalent to a garden hosepipe (but may increase to 4–5 cm diameter in shallow water to accommodate integral steel wire armouring for protection), are vulnerable to damage by external threats that can halt connections and/or significantly reduce bandwidth, requiring expensive and logistically-challenging repairs. Analysis of a global industry database revealed that approximately 150–200 cable faults occur each year, with most (60–70%) caused by human activity in <200 m water depth18. The main causes are fishing (41% of faults) and accidental anchor drops from vessels (16%). Bottom trawling is the most common type of fishing to interact with submarine cables as it occurs on most continental shelves and covers large areas of seafloor13,18,19,20. In areas of such potentially-damaging human activity, cables are buried for protection by intrusive trenching, ploughing or jetting techniques21 (Fig. 2). Cable faults caused by bottom trawling typically relate to the dragging of heavy (0.1–8 tonne) otterboards that can penetrate tens of centimeters into soft sediments and snagging of weights that are designed to stir up the seafloor to capture fish and shellfish18,22. The depletion of fishing stocks (largely driven by overfishing) has stimulated a push of demersal fishing into deeper waters in several regions15,23, meaning that cable burial is increasingly required on parts of the continental slope (in water depths up to 1500 m), in addition to the continental shelf. In some areas (e.g. north-east Atlantic and eastern Pacific Ocean), cables in up to 2000 m water depth may be buried, due to the expansion of fishing activity to greater water depths24. In deep water, where fishing and other disturbance activities are rare (i.e. less than four cable faults occur in the High Seas per year19), telecommunications cables are unarmoured and laid directly on the seafloor, causing only very minor disturbance of sediments25,26,27.

Devices include A, B plough, C jet, and D mechanical trencher. A photograph of a steep-sided 0.5 m-wide trench at 1242 m water depth is shown in E that was excavated using a jet in consolidated cohesive sediments (modified from21). Photographs A–D courtesy of Global Marine Group.

Previous studies have investigated the environmental interactions of submarine telecommunications cables, concluding that they typically exert benign to minor physical impacts on seafloor ecology25,26,27. However, it has been shown recently that disturbance by offshore human activities, such as trawling, aggregate dredging and anchoring can remove previously-buried carbon from seafloor sediments16,17,28. To date, however, no study has considered the volumes of sediment and contained organic carbon disturbed as a result of cable burial, especially considering that greater depths (i.e. up to 2 m) below seafloor than fishing activity will be disturbed. Here, we aim to assess this issue at a global scale with a view to informing more efficient management strategies to minimize future carbon release. We do this by addressing the following questions. First, what is the global footprint of seafloor disturbance by cable burial, and what is the total volume of sediment that has been disturbed by cable installation to date? Second, what volume of organic carbon has been disturbed by cable burial, and what is the likely loss of previously sequestered carbon as a result of cable burial activity? Third, how do the disturbed volumes of sediment and organic carbon compare to other natural processes and human activities? Finally, where is organic carbon more vulnerable to cable burial based on regional hotspots of reactive organic carbon?

In this present study we report a global assessment of the impacts of cable burial on sedimentary organic carbon stocks through integration of a global database that documents the extent and locations of submarine telecommunications cables, with a modelled distribution of organic carbon hosted in modern ocean sediments worldwide (trained with >11,000 sampling points)2,29. We show how up to 2.82–11.26 Mt of organic carbon worldwide has been disturbed by cable burial, and place this figure in a wider context through comparison with natural processes and other human activities.

As a base case, we assume that cables on the continental shelf (accounting for 16% of the total cable length worldwide) and on the continental slope to a water depth of 1500 m (13%) are all buried (Fig. 3). Below these water depths, some, but not all cables are buried; hence, for conservatism we also include cables lying between 1500 and 2000 m water depth (5%); the remaining cable length (i.e. 66%) is laid directly on the seafloor and is not buried. We assume a range of burial depths (0.5–2.0 m) and width of seafloor disturbance21 (0.5–1.0 m) (see Methods). Integrating these excavated dimensions indicates that the cumulative wet volume of sediment that could have been disturbed by cable burial activities to date may be as high as 0.13–1.05 km3 in water depths up to 1500 m (Table 1). Assuming disturbance extends to 2000 m water depth, yields an additional disturbed sediment volume of 0.02–0.17 km3, which makes a total of 0.15–1.22 km3 disturbed sediment (an average of 0.004–0.04 km3 per year since the start of records).

A Overview global map and zoom in to regions that feature localized hotspots including (B) South-East Asia and (C) Southern North Sea, Baltic Sea and Mediterranean Sea. Country outlines sourced from Natural Earth free vector data.

Published global estimates of sedimentary organic carbon typically focus on the top 5–10 cm below seafloor30; however, cable burial affects greater depths21. To infer the potentially disturbed stock of sedimentary carbon by cable burial activities, we use a global model that accounts for stocks within the first meter below seafloor2 (Fig. 1B). In the absence of any global dataset that extends below one meter, we necessarily assume a similar concentration of organic carbon exists to a depth of two meters (i.e. the maximum depth of cable burial assessed here). We accept this may result in an over-estimated disturbed carbon stock for that lower meter, and this data gap clearly underlines a need for greater constraint by future studies. In this model, the median of carbon stocks across continental slopes worldwide is 8632 Mg/km2, which is similar to that encountered along cable routes between 200–1500 m (8690 Mg/km2) and 1500–2000 m (9087 Mg/km2) water depth. The median value for carbon stocks across continental shelf sediments worldwide is 18,666 Mg/km2 2, yet the median value encountered along cable routes on continental shelves is less than half that value, at 8880 Mg/km2. Therefore, many of the global hotspots of sedimentary organic carbon on the continental shelf are not crossed by cable routes. We calculate disturbed carbon stocks based on mapped sedimentary carbon stocks2 along cable routes. Assuming the most conservative burial scenario of up to 2000 m water depth, the estimated volume of disturbed sediment on the continental shelf and slope equates to a disturbed sedimentary organic carbon stock of between 2.82–11.26 Mt (Table 1), of which almost equal contributions come from disturbances on the continental shelf (51%) and the slope (49%). There is considerable geographic variability in carbon stocks that may have been disturbed by cable burial activities (Fig. 3), particularly between different ocean basins (Fig. 4). The Baltic Sea stands out as the main region where cables intersect with the highest concentrations of sedimentary organic carbon, followed by the Pacific Ocean and South China and Eastern Archipelagic Seas that also feature high relative concentrations of carbon along cable routes (Fig. 3 and 4).

Sedimentary carbon stocks within top 1 m below seafloor along cable routes A shown where values are in the upper quartile of all values worldwide. Country outlines sourced from Natural Earth free vector data. Box and whisker plots B showing distribution of organic carbon stocks within top 1 m below seafloor along cable routes in different ocean basins of the world. Boxes show 25th and 75th percentiles with median annotate in between, while whiskers show full range of data.

Determining how much of those disturbed stocks are lost (i.e. oxidized and not re-sequestered) is a far more challenging issue. Disturbance of the seafloor can physically remove organic carbon through erosion, which would subsequently be re-deposited elsewhere with little or no net loss of organic carbon, whereas another fraction of organic carbon can be oxidised and degraded into aqueous carbon dioxide as a result of the exposure of sediment to the oxygenized overlying water. Quantifying the organic carbon loss through each mechanism is complicated, and studies have attempted to estimate the release of organic carbon from bottom trawling disturbance, often with contrasting results17,31,32,33,34,35. Studies focused on the continental shelf and slope in the Mediterranean and North Sea found remineralization rates of 20–60% for seafloor organic carbon disturbed by deep sea trawling17,31,32. These remineralization rates were highest in areas affected by the greatest frequency of bottom trawls; however, as cable burial is a one-off activity, the highest remineralization rates are considered to be unlikely. Speculatively assuming the lowest loss rate (i.e. 20%) from these studies, would result in a cumulative loss of 0.144–1.17 Mt of previously-buried organic carbon on the continental shelf and 0.136–1.09 Mt on the continental slope (a total of 0.280–2.25 Mt globally). However, to date no study has specifically studied the effects of cable burial on carbon disturbance at field-scale and the whether findings from bottom trawling are truly applicable to cable burial remains unclear. Consequently, there remains considerable uncertainty in the fate of sedimentary organic carbon disturbed by cable burial. First, not all excavated sediment will be released into the water column; instead most disturbed material likely rapidly resediments inside or close to the trench and thus may be effectively reburied, limiting potential for remineralization. Whether sediment rapidly backfills a trench will depend on the nature of the sediment (e.g. grain size), prevailing ocean currents near the bed and other background environmental conditions. In many cases it has been observed that trenches may refill within weeks to years on the continental shelf, but in some cases on the continental slope, where sediment supply is low this may take >15 years21. A particularly important control is likely to be the cable burial tool that is used, and the nature of the initial disturbance. In the case of ploughing and trenching, sediment typically settles quickly (particularly granular sediment, such as sand) and deposits close to the initial excavation site; in many cases immediately (fully or partially) backfilling the trench21. In such cases, the likelihood of remineralization will be reduced; however, in the case of jetting (which fluidizes the sediment), suspended plumes of fine (clay and silt-size) sediment may be more widely dispersed by ocean currents, taking days to settle and hence increasing the chances of remineralization21,36. Second, organic carbon mineralization rates will depend on external factors. For example, not all organic carbon stored in sediments is labile, and may not be remineralized after disturbance37. Previous studies have attempted to calculate a mean global oxidation rate; however, there is significant variability, due in a large part to controls exerted by ocean depth, deposition rate and primary productivity, resulting in large uncertainties38. The degradability of organic carbon, and hence remineralization rates, strongly depend on the physiographic environment and the associated chemical, biological and physical processes38,39,40. For example, regional differences in water column and sediment oxygen concentrations, and hence markedly different carbon remineralization rates, may occur in different areas, such as coastal hypoxic zones that will feature very low remineralization rates41. The rate of reactivity can vary over at least four orders of magnitude in marine sediments worldwide42. Third, areas where cables are buried may already have been extensively trawled; hence, seabed carbon stocks may already have been perturbed. Finally, cable burial differs from bottom fishing as it is intended to be a one-off activity, in contrast to fishing that leads to repeated exhumation3,15. However, in the absence of field studies related to cable burial we consider the ranges of carbon loss determined from fishing studies to be a reasonable analogue for a first-order global calculation, where most carbon loss is observed following the first fishing trawl3,15,31.

Our results indicate that to-date up to 1.22 km3 of sediment may have been disturbed during the burial of telecommunications cables (Table 1). This is of similar magnitude to that disturbed by natural processes that can exhume equivalent quantities of sediment in individual events, such as landslides that occur on the continental slope. For example, earthquake-triggered landslides can be particularly large, such as the slope failure at the head of the Kaikōura Canyon (estimated at 1.21 km3 and 7 Mt of organic carbon) following the Mw 7.8 earthquake offshore New Zealand in 201643 and the Mw 9.0 Tohoku-oki earthquake in 2011 that triggered a 0.2 km3 landslide that transported just under 1 Mt of organic carbon to 7 km water depth11. In 2020, a major river flood from the Congo River triggered an underwater avalanche that transported 2.68 km3 of sediment with 3–4% organic carbon content to 5 km water depth44. Such natural events can also be far larger, such as the >100 km3 Grand Banks landslide, which was triggered by a Mw 7.2 earthquake offshore Newfoundland in 192945. These natural disturbance events are increasingly recognized as playing an important role in the fate of sedimentary organic carbon, as they may funnel carbon to become efficiently buried in deep sea fans or hadal trenches, but may also exhume previously-buried carbon that can become remineralized11,46. A fundamental difference, however, is that such events are part of a natural spectrum that cannot be controlled, while human activities can be modified to minimize the potential for carbon disturbance.

It is increasingly apparent that human activities may exert a greater role than natural processes with regards to disturbance and remobilization of both sediment and carbon4,28,47. In 2015 alone, the global production of sediment by human activities was estimated at 150 km3, which is forecast to increase in future28; however, this value only included dredging (5.5 km3) in marine settings and neglected other offshore activities that disturb the seabed. Subsequent studies indicate that significantly larger sediment volumes (c.50 km3 annually) may be disturbed by bottom trawling; equivalent to a sedimentary disturbance of up to 21,870 Mt/year4,17,48. Our upper annual sediment disturbance estimate of 0.04 km3 by cable burial is considerably smaller than the values for these marine activities; primarily due to the smaller areal footprint (i.e. trawling grounds cover 4.9 × 106 km2,13 compared to 3–6 × 102 km2 for cable burial) despite the greater depth of penetration below seafloor of cable burial. The sedimentary organic carbon lost due to bottom trawling on an annual basis has been estimated to be >60 Mt (conservatively assuming only the top 1 cm is disturbed and 30% is lost17), which is at least two orders of magnitude greater than the cumulative total organic carbon lost due to cable burial since modern fiber-optic cables have been laid (Fig. 5). While the quantities of organic carbon lost due to cable burial are orders of magnitude smaller than associated with other human activities such as deep-sea trawling and dredging, they are non-trivial amounts that are not currently included in any global calculations and they add to the complex manner in which humans have and continue to alter natural sedimentary systems47. In light of ongoing efforts to more effectively manage marine carbon budgets, it is critical to limit disturbance of any sedimentary carbon stocks where possible. We therefore now discuss approaches that may limit such disturbance.

Note that this refers to the volumes of carbon potentially disturbed but there remains large uncertainty concerning how much of that carbon will be remineralized, and hence lost. Data based on open access records by Telegeography (https://www.submarinecablemap.com/).

Cable routes on the continental shelf generally do not cross many of the regions that host the highest sedimentary carbon stocks (Fig. 1B). This largely reflects the fact that most existing subsea cable routes occur in low to mid latitudes, while many of the high sedimentary carbon hotspots are focused in the Arctic, which is not presently a well-developed region for telecommunication cable routes49. Similarly, many other sedimentary carbon hotspots, such as offshore Namibia, Peru and Baja California, are rarely crossed. Cable routes in the Baltic Sea appear to coincide with higher than average sedimentary carbon stocks, with other notable hotspots traversed in Southeast Asia (Fig. 4). Greater constraint on the potential for the mineralization of disturbed carbon is required. In particular, the mapping of areas that are most vulnerable to carbon loss should be the focus of future studies50.

The global carbon sediment stock calculations used here are based on a machine learning model trained using n = 11,578 sediment cores, that provides an output prediction surface with a horizontal resolution of 5 × 5 arc minute (c.5–9 km). As a consequence of this relatively coarse spatial resolution, the global model does not include many very localized hotspots of sedimentary carbon enrichment. These carbon hotspots include coastal ecosystems, such as mangroves and seagrass meadows, and deep-sea submarine canyons51,52,53,54. However, organic carbon hotspots such as submarine canyons are avoided where possible for subsea cables as their irregular and steep terrain poses a hazard for routing, and they are more prone to natural hazards such as submarine landslides and sediment avalanches that can damage cables18. Of the total length of subsea cables, only 2.8% crosses submarine canyons (Fig. 1B) and are dominantly surface laid, rather than buried due to the water depths in which they lie; hence, any disturbance will be minor21. Mangroves and seagrass meadows are also avoided by cable routing where possible, primarily due to the sensitive ecology they support; however, where it is necessary to cross such areas, remedial measures can be applied, such as removal of seagrass from routes and replanting after cable emplacement, sowing of seagrass seed, or use of a custom vibrating plough in salt marshes that was shown to limit loss of sediment from the trench and where full recovery occurred within five years55. Directional drilling has been used to install cables beneath sensitive coastal areas to completely avoid any disturbance56. Blue Carbon systems such as seagrass meadows and mangroves are not incorporated in the global model used here; hence, more local assessments would assist with route planning. Near-seafloor deposits with extremely high organic carbon contents, such as buried peats in the North Sea (up to 50% total organic carbon content) may be especially vulnerable to disturbance; yet little work has been done to constrain their extent or the consequences of large losses of irrecoverable carbon from these long-term stores57,58,59. Past peaks in demand for digital connectivity saw the installation of greater numbers and lengths of subsea cables (e.g. the late 1990s "dot-com boom"; Fig. 5). Future demand for high bandwidth connectivity may therefore see a similar expansion of the submarine cable network, including new routes in areas that may feature high sedimentary carbon stocks. We suggest that the potential disturbance of sedimentary carbon stocks, and particularly minimizing the impacts on carbon hotspots, should be a consideration when planning cable routes, in a similar manner to the assessment of natural hazards, human activities and environmental impacts.

Given the minor environmental impacts of surface-laid cables25,26,27, it could be tempting to suggest that cable burial is avoided to minimize disturbance of sedimentary carbon stocks. However, the primary role of cable burial is for protection against human activities, in particular bottom trawling. Indeed, the main reason cables are buried in water depths up to 2000 m is due to the expansion of bottom fishing. If bottom fishing efforts were reduced (e.g. limiting the depth of penetration of otterboards) or restricted close to cable routes, this would provide a two-fold benefit for the preservation of sedimentary carbon: (i) limiting the depth and intensity of disturbance by bottom trawling60,61; and in addition, (ii) reducing the depth or avoiding the need for cable burial entirely in deep water. A cable protection zone in the Cook Strait, New Zealand covers an area of 236 km2 within which neither anchoring nor fishing are permitted36. Such initiatives elsewhere may provide mutual benefits for cable resilience and for marine ecology as they can create a reserve effect by restricting other human activities.

Unlike deep sea fishing, that may repetitively affect a fishing ground, cable burial is intended to be a one-off activity; hence, any disturbance is generally restricted to that initial period, with the exception being unpredictable and rare instances where repair is required. Historically, when they reach the end of their operational life, cables have been left in place due to their inert nature. Indeed, recovered sections of cables from the Pacific and Atlantic have been observed to be near-pristine and physically intact after almost 50 years62. This limited degradation supports the case for leaving cables in place; however, these same properties make decommissioned cables potentially high-grade recycling targets, particularly the polyethylene plastic, steel and copper components. While this is potentially a valuable contribution to enhanced sustainability, assessing whether a decommissioned cable is to be recycled or left in place should carefully consider any adverse impacts its recovery may have on the seafloor environment, benthic communities21 and sedimentary carbon stocks. While the volumes of sediment and carbon disturbance that we estimate in this study may be significantly smaller than activities such as fishing, it is important to constrain the effects of any human activity that may disturb the sedimentary carbon stocks and provide marine-planning guidance to minimize disturbance where it is possible. The impacts to carbon stores have largely been ignored for other human activities that involve excavation of seafloor sediments, such as the burial of oil and gas pipelines, cables that transfer electricity, and large diameter foundations for offshore renewables structures, but should be considered in future, particularly as infrastructure developments extend into sedimentary carbon hotspots such as the Arctic49.

The growing demand for high-bandwidth international communications and data transfer means that the global network of subsea cables continues to grow rapidly, including new routes to previously-unconnected regions and enhancement of existing connections. These connections play a critical role in sustainable development, and reduce reliance on both domestic and international travel. While the physical environmental impact of these cables is relatively minor, we have shown that the total volume of sedimentary disturbance due to cable burial (albeit over multiple decades) can be equivalent to that exhumed during major natural disturbances, such as a submarine landslide. We highlight opportunities to minimize carbon loss for future cable routes and propose that restricting fishing activity near cable routes may have a two-fold benefit, both diminishing carbon lost due to bottom trawling and reducing or avoiding the need to bury cables in deep water. This study presents a global assessment of the sedimentary carbon that may have been disturbed by cable burial, but the uncertainties in our estimates underline a pressing need for field and laboratory-based calibration studies to determine the fate of disturbed organic carbon. Such studies are essential to constrain organic carbon disturbance and loss across a wide range of water depths, and diverse physiographic and oceanographic settings, to quantify the true loss and vulnerability of sedimentary organic carbon to human activities.

The depth and width of seafloor disturbance depends upon the requirements for protection, the nature of the seafloor substrate, and the type of cable burial technique that is used (Fig. 2). On the basis of prior studies, including field studies of pre- and post-installation disturbance, we assume a range of credible burial depths (0.5–2.0 m) and width of seafloor disturbance (0.5–1.0 m) based on published values18,21. We now provide details on the different types of cable burial technique.

Ploughing involves simultaneously laying and burying a cable and is a widely used technique21. Ploughs are towed by a cable laying ship and include an assembly mounted on skids or caterpillar tracks from which a narrow furrow is excavated by a blade (‘plough share’) to the desired depth of burial, which can be up to 3 m below seafloor. Excavated sediment is then allowed to fall back in and infill the furrow. The physical width of the largest commercial plough share that cuts the furrow is 45 cm, (SubCom, Pers. Comm.), but is typically 30 cm55. The plough skids on either side of the furrow may be up to 75 cm wide, and may compress seafloor sediments, the extent to which that occurs depends on their stiffness. The final width of the excavated furrow also depends on sediment type. For example, a 45 cm-wide furrow might widen in poorly consolidated sediments if the sidewalls of the furrow collapse into the furrow itself. In cohesive sediments, the furrow width is likely equal to the width of the plough share, however, in granular sediments the width may be slightly wider (<1 m21). Ploughing is limited by the length of umbilical and tow wire to a maximum of 1200–1500 m water depth, where the plough becomes hard to control, especially coming down or up slopes over the continental shelf break.

Jetting is typically used where seabed conditions are unfavourable for ploughing, such as on steep slopes or in water depths beyond 1200–1500 m, although the latter case is rare21. This technique involves fluidization of the seafloor beneath a "sword" that is deployed from a Remotely Operated Vehicle that may extend to a depth of 3 m below seafloor, to fluidize a 15–30 cm-wide area. The cable is covered by sediment that settles out from the fluidized slurry. Jetting in cohesive sediments tends to create steep-sided profiles, while broader profiles are more common in granular sediments. Observed seafloor disturbance widths are <1 m, and typically much narrower21. Jetting disturbs the sediment far more than ploughing, and may create berms of sand and gravel close to the trench (<100 m) and can disperse suspended mud more widely (up to 2 km21).

Less common is the use of mechanical trenchers, where a tracked vehicle uses a mechanical chain excavator or rock cutters to excavate a trench in areas of rocky seafloor. Trenching may reach depths of up to 1.5–3 m below seafloor, with widths of <1 m. Cable routing generally avoids areas of rocky substrate, and given the expense and environmental impacts of trenching, this is a last resort. In many cases where rocky seafloors must be traversed, cables are laid in protective casing on the seafloor rather than attempting burial21,27. In nearshore zones where cable burial may not be possible due to challenging substrates or due to the presence of sensitive habitats (e.g. seagrass, mangroves) horizontal directional drilling may be used. This approach involves the subsurface drilling of a hole, through which a cable is passed and avoids any seafloor disturbance21.

The total length of submarine telecommunications cables was determined by summing the total length of all of the individually identified cable sections in a proprietary database provided for this project by Global Marine Ltd. This database details precise cable locations, including operational cables and those that have been decommissioned (out-of-service cables). Cross-checking this length against an open-access database of cable lengths (Telegeography: https://www.submarinecablemap.com/), indicates a difference of less than 3%, with a total length calculated from the Global Marine database of 1.82 × 106, compared to 1.88 × 106 from Telegeography. Of the total length in the Global Marine database, 13.6% of the cable length (2.47 × 105) was reported to be out-of-service as of the December 2020. As the Telegeography database does not provide precise location information, we necessarily use the Global Marine database to calculate the length of cable that requires burial. An estimated 13.5% of the total length lies within Areas Beyond National Jurisdiction.

In order to calculate the volume of sediment disturbed by cable burial activities, we first determine the length of cables that are laid in water depths where burial is required. We use the 2022 GEBCO bathymetric map of the oceans (GEBCO, 2022) to determine water depths along each of the cable routes in the Global Marine database. We first excluded all cable lengths that lie in water depths >2000 m. We then differentiated by cable lengths that lie on the continental shelf, the continental shelf between to water depth of 1500 m, and between 1500 m and 2000 m (based on the World Seafloor Geomorphology map of GRID Arendal63. We make this differentiation because cables are typically buried to water depths of up to 1500 m, but in some regions (particularly the NE Atlantic) burial is sometimes required to 2000 m water depth. In so doing, we aim to provide a conservative upper bound (i.e. including water depths of up to 2000 m). We then relate these cable lengths to the dimensions of the trenches excavated for cable burial, which provide upper and lower bounds for the potentially disturbed volume of sediment. Disturbed seabed area is derived by multiplying cable length by trench width (0.5–1.0 m), and then related to disturbed sediment volume by multiplying that value by trench depth (0.5–2.0 m). Finally, we relate the disturbed sediment volumes to the global modeled sedimentary carbon stocks of Atwood et al.2. We do this in two ways. First we simply base this on global average values of carbon/km2 within the top 1 m below seafloor that Atwood et al. provide for the continental shelf and continental slope. Second, we use the mapped values of carbon/km2 from the global model of Atwood (i.e. Fig. 2B), extracting the values along each cable route to enable a more geographically-resolved calculation. Where we assume a burial depth scenario of 0.5 m, we half this value, and for a burial depth of 2 m, we double the value.

We calculate the length of cables crossing the main physiographic domains in the ocean, as well as key marine biodiversity hotspots, i.e. submarine canyons, by clipping the cable polylines in ArcGIS version 10.8 to the extent of shapefiles from the World Seafloor Geomorphology map of GRID Arendal,

Bathymetric data from which water depths of cable route sections were extracted are reproduced from the GEBCO_2022 Grid, GEBCO Compilation Group (2022) GEBCO 2022 Grid (https://doi.org/10.5285/e0f0bb80-ab44-2739-e053-6c86abc0289c) and can be accessed at https://www.gebco.net/data_and_products/gebco_web_services/web_map_service/. The World Seafloor Geomorphological map from which extents of continental shelves, continental slope and other physiographic domains were calculated can be accessed at https://www.arcgis.com/home/item.html?id=3a40d6b0035d4f968f2621611a77fe64. While the database of precise cable locations used for our analysis is proprietary, the approximate location of cable routes can be downloaded at https://github.com/telegeography/www.submarinecablemap.com. The global mapping of sedimentary organic carbon used in our analysis is available at https://figshare.com/articles/dataset/Global_marine_sedimentary_carbon_stock/11956356. Country outlines were sourced from Natural Earth and version 5.1.1. of the 10 m resolution open access dataset can be downloaded from https://github.com/nvkelso/natural-earth-vector/find/master.

Estes, E. R. et al. Persistent organic matter in oxic subseafloor sediment. Nat. Geosci. 12, 126 (2019).

Article ADS CAS Google Scholar

Atwood, T. B., Witt, A., Mayorga, J., Hammill, E. & Sala, E. Global patterns in marine sediment carbon stocks. Front. Mar. Sci. 7, 165 (2020).

Article Google Scholar

Legge, O. et al. Carbon on the northwest European shelf: contemporary budget and future influences. Front. Mar. Sci. 7, 143 (2020).

Article Google Scholar

Sala, E. et al. Protecting the global ocean for biodiversity, food and climate. Nature 592, 397–402 (2021).

Article ADS CAS PubMed Google Scholar

Pendleton, L. et al. Estimating global "blue carbon" emissions from conversion and degradation of vegetated coastal ecosystems. PLoS One 7, e43542 (2012).

Article ADS CAS PubMed PubMed Central Google Scholar

Lovelock, C. E. et al. Assessing the risk of carbon dioxide emissions from blue carbon ecosystems. Front. Ecol. Environ. 15, 257–265 (2017).

Article Google Scholar

Pusceddu, A. et al. Impact of natural (storm) and anthropogenic (trawling) sediment resuspension on particulate organic matter in coastal environments. Continental Shelf Res. 25, 2506–2520 (2005).

Article ADS Google Scholar

Ferré, B., De Madron, X. D., Estournel, C., Ulses, C. & Le Corre, G. Impact of natural (waves and currents) and anthropogenic (trawl) resuspension on the export of particulate matter to the open ocean: application to the Gulf of Lion (NW Mediterranean). Continental Shelf Res. 28, 2071–2091 (2008).

Article ADS Google Scholar

Allison, M. A., Dellapenna, T. M., Gordon, E. S., Mitra, S. & Petsch, S. T. Impact of Hurricane Katrina (2005) on shelf organic carbon burial and deltaic evolution. Geophys. Res. Lett. 37, 21 (2010).

Article Google Scholar

Pastor, L. et al. Transient early diagenetic processes in Rhône prodelta sediments revealed in contrasting flood events. Continental Shelf Res. 166, 65–76 (2018).

Article ADS Google Scholar

Kioka, A. et al. Megathrust earthquake drives drastic organic carbon supply to the hadal trench. Sci. Rep. 9, 1–10 (2019).

Article CAS Google Scholar

Doney, S. C. The growing human footprint on coastal and open-ocean biogeochemistry. Science 328, 1512–1516 (2010).

Article ADS CAS PubMed Google Scholar

Kroodsma, D. A. et al. Tracking the global footprint of fisheries. Science 359, 904–908 (2018).

Article ADS CAS PubMed Google Scholar

Bugnot, A. B. et al. Current and projected global extent of marine built structures. Nat. Sustainability 4, 33–41 (2021).

Article Google Scholar

Paradis, S. et al. Bottom-trawling along submarine canyons impacts deep sedimentary regimes. Sci. Rep. 7, 1–12 (2017).

CAS Google Scholar

Watson, S. J. et al. The footprint of ship anchoring on the seafloor. Sci. Rep. 12, 1–11 (2022).

Article Google Scholar

Paradis, S. et al. Persistence of biogeochemical alterations of deep‐sea sediments by bottom trawling. Geophys. Res. Lett. 48, e2020GL091279 (2021).

Article ADS CAS Google Scholar

Carter, L. Submarine cables and the oceans: connecting the world (No. 31). UNEP/Earthprint (2010).

Burnett, D. R. & Carter, L. International submarine cables and biodiversity of areas beyond national jurisdiction: the cloud beneath the sea (p. 80). Brill (2017).

Amoroso, R. O. et al. Bottom trawl fishing footprints on the world's continental shelves. Proc. Natl Acad. Sci. USA 115, E10275–E10282 (2018).

Article ADS CAS PubMed PubMed Central Google Scholar

Kraus, C. & Carter, L. Seabed recovery following protective burial of subsea cables-observations from the continental margin. Ocean Eng. 157, 251–261 (2018).

Article Google Scholar

Løkkeborg, S. Impacts of trawling and scallop dredging on benthic habitats and communities (Vol. 472). Food & Agriculture Organisation of the United Nations (2005).

Watson, R. A. & Morato, T. Fishing down the deep: accounting for within-species changes in depth of fishing. Fish. Res. 140, 63–65 (2013).

Article Google Scholar

Benn, A. R. et al. Human activities on the deep seafloor in the North East Atlantic: an assessment of spatial extent. PLoS ONE 5, e12730 (2010).

Article ADS PubMed PubMed Central Google Scholar

Andrulewicz, E., Napierska, D. & Otremba, Z. The environmental effects of the installation and functioning of the submarine SwePol Link HVDC transmission line: a case study of the Polish Marine Area of the Baltic Sea. J. Sea Res. 49, 337–345 (2003).

Article ADS Google Scholar

Kogan, I. et al. ATOC/Pioneer Seamount cable after 8 years on the seafloor: observations, environmental impact. Continental Shelf Res. 26, 771–787 (2006).

Article ADS Google Scholar

Sherwood, J. et al. Installation and operational effects of a HVDC submarine cable in a continental shelf setting: Bass Strait, Australia. J. Ocean Eng. Sci. 1, 337–353 (2016).

Article Google Scholar

Cooper, A. H., Brown, T. J., Price, S. J., Ford, J. R. & Waters, C. N. Humans are the most significant global geomorphological driving force of the 21st century. Anthropocene Rev. 5, 222–229 (2018).

Article Google Scholar

van der Voort, T. S. et al. MOSAIC (Modern Ocean Sediment Archive and Inventory of Carbon): a (radio) carbon-centric database for seafloor surficial sediments. Earth Syst. Sci. Data 13, 2135–2146 (2021).

Article ADS Google Scholar

Lee, T. R., Wood, W. T. & Phrampus, B. J. A machine learning (kNN) approach to predicting global seafloor total organic carbon. Glob. Biogeochemical Cycles 33, 37–46 (2019).

Article ADS CAS Google Scholar

De Borger, E., Tiano, J., Braeckman, U., Rijnsdorp, A. D. & Soetaert, K. Impact of bottom trawling on sediment biogeochemistry: a modelling approach. Biogeosciences 18, 2539–2557 (2021).

Article ADS Google Scholar

Morys, C., Brüchert, V. & Bradshaw, C. Impacts of bottom trawling on benthic biogeochemistry in muddy sediments: Removal of surface sediment using an experimental field study. Mar. Environ. Res. 169, 105384 (2021).

Article CAS PubMed Google Scholar

Mayer, L. M., Schick, D. F., Findlay, R. H. & Rice, D. L. Effects of commercial dragging on sedimentary organic matter. Mar. Environ. Res. 31, 249–261 (1991).

Article Google Scholar

Tiano, J. C. et al. Acute impacts of bottom trawl gears on benthic metabolism and nutrient cycling. ICES J. Mar. Sci. 76, 1917–1930 (2019).

Article Google Scholar

Trimmer, M. et al. Impact of long-term benthic trawl disturbance on sediment sorting and biogeochemistry in the southern North Sea. Mar. Ecol. Prog. Ser. 298, 79–94 (2005).

Article ADS CAS Google Scholar

Taormina, B. et al. A review of potential impacts of submarine power cables on the marine environment: knowledge gaps, recommendations and future directions. Renew. Sustain. Energy Rev. 96, 380–391 (2018).

Article CAS Google Scholar

Keil, R. G., Montluçon, D. B., Prahl, F. G. & Hedges, J. I. Sorptive preservation of labile organic matter in marine sediments. Nature 370, 549–552 (1994).

Article ADS Google Scholar

Jørgensen, B. B., Wenzhöfer, F., Egger, M. & Glud, R. N. Sediment oxygen consumption: role in the global marine carbon cycle. Earth-Sci. Rev. 228, 103987 (2022).

Article Google Scholar

Stolpovsky, K., Dale, A. W. & Wallmann, K. A new look at the multi-G model for organic carbon degradation in surface marine sediments for coupled benthic–pelagic simulations of the global ocean. Biogeosciences 15, 3391–3407 (2018).

Article ADS CAS Google Scholar

Arndt, S. et al. Quantifying the degradation of organic matter in marine sediments: a review and synthesis. Earth-Sci. Rev. 123, 53–86 (2013).

Article ADS CAS Google Scholar

Lichtschlag, A. et al. Effects of fluctuating hypoxia on benthic oxygen consumption in the Black Sea (Crimean shelf). Biogeosciences 12, 5075–5092 (2015).

Article ADS Google Scholar

Freitas, F. S. et al. New insights into large-scale trends of apparent organic matter reactivity in marine sediments and patterns of benthic carbon transformation. Biogeosciences 18, 4651–4679 (2021).

Article ADS CAS Google Scholar

Mountjoy, J. J. et al. Earthquakes drive large-scale submarine canyon development and sediment supply to deep-ocean basins. Sci. Adv. 4, eaar3748 (2018).

Article ADS PubMed PubMed Central Google Scholar

Talling, P. J. et al. Longest sediment flows yet measured show how major rivers connect efficiently to deep sea. Nat. Commun. 13, 4193 (2022).

Article ADS CAS PubMed PubMed Central Google Scholar

Schulten, I., Mosher, D. C., Piper, D. J. & Krastel, S. A massive slump on the St. Pierre Slope, a new perspective on the 1929 Grand Banks submarine landslide. J. Geophys. Res.: Solid Earth 124, 7538–7561 (2019).

Article ADS Google Scholar

Azpiroz-Zabala, M. et al. Newly recognized turbidity current structure can explain prolonged flushing of submarine canyons. Sci. Adv. 3, e1700200 (2017).

Article ADS PubMed PubMed Central Google Scholar

Syvitski, J. et al. Earth's sediment cycle during the Anthropocene. Nat. Rev. Earth Environ. 3, 1–18 (2022).

Oberle, F. K., Storlazzi, C. D. & Hanebuth, T. J. What a drag: quantifying the global impact of chronic bottom trawling on continental shelf sediment. J. Mar. Syst. 159, 109–119 (2016).

Article Google Scholar

Clare, M. A. et al. Climate change hotspots and implications for the global subsea telecommunications network. Earth-Sci. Rev. 237, 104296 (2022).

Tye, A. M., Evans, D. L., Lee, J. R. & Robinson, D. A. The role of post UK-LGM erosion processes in the long-term storage of buried organic C across Great Britain–A ‘first order’assessment. Earth-Science Reviews, p.104126 (2022).

Masson, D. G. et al. Efficient burial of carbon in a submarine canyon. Geology 38, 831–834 (2010).

Article ADS CAS Google Scholar

Taillardat, P., Friess, D. A. & Lupascu, M. Mangrove blue carbon strategies for climate change mitigation are most effective at the national scale. Biol. Lett. 14, 20180251 (2018).

Article PubMed PubMed Central Google Scholar

De Leo, F. C., Smith, C. R., Rowden, A. A., Bowden, D. A. & Clark, M. R. Submarine canyons: hotspots of benthic biomass and productivity in the deep sea. Proc. R. Soc. B: Biol. Sci. 277, 2783–2792 (2010).

Article Google Scholar

Clark, M. R., Watling, L., Rowden, A. A., Guinotte, J. M. & Smith, C. R. A global seamount classification to aid the scientific design of marine protected area networks. Ocean Coast. Manag. 54, 19–36 (2011).

Article Google Scholar

Carter, L., Burnett, D. & Davenport, T. The relationship between submarine cables and the marine environment. In Submarine Cables (pp. 179–212). Brill Nijhoff (2014).

Austin, S., Wyllie-Echeverria, S. & Groom, M. J. A comparative analysis of submarine cable installation methods in Northern Puget Sound, Washington. J. Mar. Environ. Eng. 7, (2004).

Lippmann, T. J. et al. Microbial activity, methane production, and carbon storage in Early Holocene North Sea peats. Biogeosciences 18, 5491–5511 (2021).

Article ADS CAS Google Scholar

Cartelle, V. et al. Sedimentary architecture and landforms of the late Saalian (MIS 6) ice sheet margin offshore of the Netherlands. Earth Surf. Dyn. 9, 1399–1421 (2021).

Article ADS Google Scholar

Harris, L. I. et al. The essential carbon service provided by northern peatlands. Front. Ecol. Environ. 20, 222–230 (2022).

Article Google Scholar

Sciberras, M. et al. Response of benthic fauna to experimental bottom fishing: a global meta‐analysis. Fish. Fish. 19, 698–715 (2018).

Article Google Scholar

Hiddink, J. G. et al. Global analysis of depletion and recovery of seabed biota after bottom trawling disturbance. Proc. Natl Acad. Sci. USA 114, 8301–8306 (2017).

Article ADS CAS PubMed PubMed Central Google Scholar

Carter, L., Collins, K., Creese, C. & Waterworth, G. Chemical and physical stability of submarine fibre-optic cables in the Area Beyond National Jurisdiction (ABNJ). SubOptic 2019 (2020).

Harris, P. T., Macmillan-Lawler, M., Rupp, J. & Baker, E. K. Geomorphology of the oceans. Mar. Geol. 352, 4–24 (2014).

Article ADS Google Scholar

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MAC acknowledges funding from the Natural Environment Research Council (NERC) National Capability Programme (NE/R015953/1) "Climate Linked Atlantic Sector Science" and from the International Cable Protection Committee. We thank Global Marine Ltd for providing access to a global database of telecommunications cables.

Ocean Biogeoscience Research Group, National Oceanography Centre, Southampton, UK

M. A. Clare & A. Lichtschlag

Geological Institute, ETH Zürich, Zürich, Switzerland

S. Paradis

School of Earth and Environment, University of Leeds, Leeds, UK

N. L. M. Barlow

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The study was conceived by M.A.C. Analysis was led by M.A.C. A.L., S.P., and N.L.M.B. contributed to the development, writing and editing of the manuscript.

Correspondence to M. A. Clare.

M.A.C. acknowledges financial support from the International Cable Protection Committee that supported the analysis of the spatial footprint of subsea cables and the extent of their burial. All other authors declare no competing interests.

Nature Communications thanks Cai Ladd, Paul Lundgren and the anonymous reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Clare, M.A., Lichtschlag, A., Paradis, S. et al. Assessing the impact of the global subsea telecommunications network on sedimentary organic carbon stocks. Nat Commun 14, 2080 (2023). https://doi.org/10.1038/s41467-023-37854-6

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Received: 30 August 2022

Accepted: 28 March 2023

Published: 12 April 2023

DOI: https://doi.org/10.1038/s41467-023-37854-6

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