Abstract
Particle fluxes at the Southern Ocean time series (SOTS) site in the Subantarctic zone (SAZ) south of Australia (~47°S, 142°E, 4600m water depth) were collected from 1997 – 2017 using moored sediment traps at nominal depths of 1000m, 2000m and 3800m. Annually integrated mass fluxes showed moderate variability of 14 ± 6 g m-2 yr-1 at 1000m, 20 ± 6 g m-2 yr-1 at 2000m and 21 ± 4 g m-2 yr-1 at 3800m. Particulate organic carbon (POC) fluxes were similar to the global median, indicating that the Subantarctic Southern Ocean exports considerable amounts of carbon to the deep sea despite its high-nutrient, low chlorophyll characteristics. The interannual flux variations were larger than those of net primary productivity as estimated from satellite observations. Particle compositions were dominated by carbonate minerals (> 60% at all depths), opal (~10% at all depths), and particulate organic matter (~17% at 1000m, decreasing to ~10% at 3800m), with seasonal and interannual variability much smaller than for their flux magnitudes. The carbonate counter-pump effect reduced carbon sequestration by ~8±2%. The average seasonal cycle at 1000m had a two-peak structure, with a larger early spring peak (October/November) and a smaller late summer (January/February) peak. At the two deeper traps, these peaks became less distinct with a greater proportion of the fluxes arriving in autumn. Singular value decomposition (SVD) shows that this temperate seasonal structure accounts for ~80% of the total variance (SVD Mode 1), but also that its influence varies significantly relative to Modes 2 and 3 which describe changes in seasonal timings. This occurrence of significant interannual variability in seasonality yet relatively constant annual fluxes, is likely to be useful in selecting appropriate models for the simulation of environmental-ecological coupling and its role in controlling the biological carbon pump. No temporal trends were detected in the mass or component fluxes, or in the time series of the SVD Modes. The SOTS observations provide an important baseline for future changes expected to result from warming, stratification, and acidification in this globally significant region.
Highlights
The constant sinking of particles from the euphotic zone moves carbon away from the atmosphere and connects the surface and deep sea on shorter timescales than achieved by advection (Agassiz, 1888; Boyd and Trull, 2007; Buesseler et al, 2007a)
We present a 20-year time series of particle fluxes collected by deep ocean sediment traps in the Subantarctic Zone (SAZ) of the Southern Ocean south of Australia, in terms of dry mass and three main chemical components: particulate organic matter (POM), expressed as particulate organic carbon (POC), and two main biogenic ballasting materials, calcium carbonate, expressed as particulate inorganic carbon (PIC), and opal, expressed as biogenic silica (BSi)
The Southern Ocean time series is a Sub-Facility of the Australian Integrated Marine Observing System (IMOS), which currently consists of two deep ocean moorings: the Southern Ocean Flux Station (SOFS) and the SAZ sediment trap mooring
Summary
The constant sinking of particles from the euphotic zone moves carbon away from the atmosphere and connects the surface and deep sea on shorter timescales than achieved by advection (Agassiz, 1888; Boyd and Trull, 2007; Buesseler et al, 2007a). The Southern Ocean has been estimated to account for ∼30% of the global annual oceanic carbon export, i.e., about 3 Pg C yr−1 (Arteaga et al, 2018), even though it accounts for less than 20% of the global ocean surface area Given this large contribution, and the status of the Southern Ocean as the largest region with unused surface ocean macro-nutrients and the greatest capacity for increased biological carbon pump strength (e.g., Boyd et al, 2000; Trull et al, 2001a), it is paramount to resolve the question of how global climate change will alter this region’s ability and efficiency to take up atmospheric CO2 (Falkowski et al, 2000; Cram et al, 2018). Sediment traps are widely used for this purpose and they have their limitations in terms of providing quantitative flux estimates, especially at shallow depths, as extensively reviewed by Yu et al (2001) and Buesseler et al (2007a), they are currently the best available tool for collecting year round particle flux data
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