How turbidity currents dictate organic carbon fluxes across river-fed fjords
S. Hage1,2,3*, V.V. Galy4, M.J.B. Cartigny5, C. Heerema6,5, M.S. Heijnen7, S. Acikalin8, M.A. Clare7, I. Giesbrecht9,10, D.R. Gröcke11, A. Hendry8, R. G. Hilton12, S.M. Hubbard2, J.E. Hunt7, D.G. Lintern13, C. McGhee8, D.R. Pasons14, E. L. Pope5, C D. Stacey13, E.J. Sumner3, S. Tank15,9, P.J. Talling5,11
1Univ Brest, CNRS, Ifremer, Geo-Ocean, F-29280 Plouzané, France
2Department of Geoscience, University of Calgary, Canada
3School of Ocean and Earth Sciences, University of Southampton, UK
4Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution
5Department of Geography, Durham University, UK
6Department of Geography, University of Victoria, Victoria, BC, Canada V8W 2Y2
7National Oceanography Centre Southampton, UK
8School of Natural and Environmental Sciences, Newcastle University, UK
9Hakai Institute, Vancouver, BC, Canada
10School of Resource and Environmental Management, Simon Fraser University, Burnaby, BC, Canada
11Department of Earth Sciences, Durham University, UK
12Department of Earth Sciences, University of Oxford, UK
13Geological Survey of Canada, Natural Resources Canada, Sidney, BC, Canada
14Energy and Environment Institute, University of Hull, UK
15Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
*Corresponding author: Sophie Hage (sophie.hage@ifremer.fr)
Abstract
The delivery and burial of terrestrial particulate organic carbon (OC) in marine sediments is important to quantify, because this OC is a food resource for benthic communities, and if buried it may lower the concentrations of atmospheric CO2 over geologic timescales. Analysis of sediment cores has previously shown that fjords are hotspots for OC burial. Fjords can contain complex networks of submarine channels formed by seafloor sediment flows, called turbidity currents. However, the burial efficiency and distribution of OC by turbidity currents in river-fed fjords had not been investigated previously. Here, we determine OC distribution and burial efficiency across a turbidity current system within a fjord, in Bute Inlet (Canada). We show that 60 ± 10 % of the OC supplied by the two river sources, is buried across the fjord surficial (2 m) sediment. The sand-dominated submarine channel and its terminal lobe contain 63 ± 14 % of the annual terrestrial OC burial in the fjord. In contrast, the muddy overbank and distal flat basin settings contain the remaining 37 ± 14 %. OC in the channel, lobe and overbank exclusively comprises terrestrial OC sourced from rivers. When normalized by the fjord’s surface area, at least three times more terrestrial OC is buried in Bute Inlet, compared to the muddy parts of other fjords previously studied. Although the long-term (>100 year) preservation of this OC is still to be fully understood, turbidity currents in fjords appear to be efficient in storing OC supplied by rivers in their near-surface deposits.
Plain Language summary
Plants on land use CO2 in the atmosphere to produce organic carbon, which promotes their growth. Rivers transport organic carbon to the sea, where it is either eaten by fauna or buried in the seafloor, thus decreasing CO2 levels on Earth over thousands to millions of years. Fjords are recognized as global organic carbon sinks; trapping 18 million tons of organic carbon in their seafloor each year. However, the complex morphology of fjord seafloors was not considered in the calculation of this organic carbon flux. In this study we determine the distribution and abundance of organic carbon across a fjord (Bute Inlet, Canada), which contains a submarine channel network terminating onto a large accumulation of sand (called lobe). We show that 60 % of the organic carbon supplied by the two rivers connected to the fjord is buried across the fjord; the majority of this carbon being held in the channel and lobe. In total, Bute Inlet buries at least three times more organic carbon per surface area than other fjords previously studied. Submarine channels in fjords thus appear to promote the storage of land-derived organic carbon in the seafloor, potentially impacting CO2 levels and food resources for marine fauna.
1. Introduction
The sequestration of terrestrial particulate organic carbon (OC) in marine sediments can lead to a drawdown of atmospheric CO2, and thus form a long-term control on carbon dioxide and oxygen levels in Earth’s atmosphere (Berner, 1982, Burdige, 2007, Hilton and West, 2020). Terrestrial OC also constitutes a key energy resource for benthic communities living on the seabed, including in fjords (Hunter et al., 2013, Włodarska‐Kowalczuk et al., 2019). It is therefore important to constrain the fluxes of terrestrial OC delivered to the seabed over short (decades to centuries) and long (thousand to million year) timescales. Major river deltas worldwide bury about 47 Mt of terrestrial OC each year, approximately 30 % of the total OC burial including marine organic matter in the oceans (Berner, 1989; Hedges & Keil, 1995; Burdige, 2007). Despite their surface areas being 40 times smaller than that of deltas and shelves, fjords have been shown to represent 17 % of the global terrestrial OC burial (Cui et al., 2016). Therefore, fjord systems are hotspots for OC sequestration (Smith et al., 2015, Cui et al., 2016, 2017). However, these global OC burial estimates in fjords are based on samples taken predominantly from the muddy parts of fjords, and assume the seafloor in fjords is homogeneous in terms of OC burial and sedimentation rates.
Fjord seafloors can, however, be highly heterogeneous (Smeaton and Austin, 2019; Bianchi et al., 2020) and comprise diverse sub-environments including submarine channels, and associated overbanks and lobes (Zeng et al., 1991, Conway et al., 2012, Pope et al., 2019). Such sub-environments are created by submarine sediment density flows, called turbidity currents. These flows distribute sediment and OC across the fjord floor, connecting river mouths to the deeper parts of fjords, and fractionate sediment and OC by grain size and density en-route. Only a handful of studies have discussed how OC is distributed and fractionated within fjords (Cui et al., 2016, 2017; Smeaton and Austin, 2020; Bianchi et al., 2020; Hage et al., 2020). For example, a recent study of Scottish fjords revealed that muddy (<63 µm) sediments held the largest amounts of OC (~2.6 Mt) compared to sandy sediment (~0.26 Mt; Smeaton and Austin, 2020). While the OC budgets are known in the Scottish fjords, the processes determining these budgets remain uncertain, partly because of a lack of bathymetric data and knowledge of submarine flow processes in these settings (e.g. these Scottish fjords may not contain active turbidity currents). In contrast, a study of Bute Inlet (a fjord in British Columbia, Canada) highlighted that the sandy parts of a submarine channel formed by turbidity currents, held large amounts of OC, particularly compared to the overlying mud-rich sediments (Hage et al., 2020). However, the river-derived OC inputs and burial rates in different sub-environments created by turbidity currents were not considered in that previous study of Bute Inlet, nor in any other fjord. Given the large variation and uncertainty in OC burial efficiency between fjord sub-environments, it is important to assess this variability more accurately to estimate global OC budgets (Burdige, 2007, Smith et al., 2015) and to improve paleoclimate/environmental reconstructions based on sediment cores in fjords (Bianchi et al., 2020).
Here for the first time, we quantify the amount and type of OC supplied by two rivers and its distribution in the surficial sediments (top two meters) of different turbidity current sub-environments (e.g. channel floor, overbanks, lobe, distal basin) across a river-fed fjord. This study then shows how OC is unevenly distributed within a fjord, and illustrates how OC distribution and burial is dependent on turbidity current processes. Insights into how OC is distributed have wider implications for understanding OC burial by turbidity currents in locations other than fjords, for which there are also very few detailed OC budgets from river sources to marine sinks. Therefore, we compare our Bute Inlet results with available information on how OC is distributed in sandier or muddier sub-environments of other turbidity current systems, i.e. Gaoping (Kao et al., 2014, Liu et al., 2016), Ganges-Brahmaputra (Galy et al., 2007, Lee et al., 2019), Congo (Baudin et al, 2020). This comparison highlights whether similar or contrasting OC burial patterns emerge more generally.
The main objective of this paper is to constrain OC fluxes from source-to-sink in Bute Inlet by answering the following questions: 1) How much OC is delivered by the two rivers discharging into Bute Inlet? 2) How much and what type of OC is present in the Bute Inlet sub-environments? 3) What is the terrestrial OC burial efficiency of Bute Inlet and how does it compare with other fjords? Finally, we discuss the study in a wider global context, and thus compare our results from Bute Inlet with (non-fjord) deep-sea turbidity current systems.
2. Bute Inlet: a fjord fed by two rivers
Bute Inlet is a 78 km-long fjord in British Columbia, Canada (Fig. 1). The inlet has an average width of ~4 km, an average depth of 550 m (with a maximum of 660 m; Prior et al., 1987), and a total surface area of 273 km2 (including the fjord’s steep sidewalls).