Increased sea level rise accelerates carbon sequestration in a macro-tidal salt marsh.

Salt marshes are known as key ecosystems for nature-based climate mitigation through organic carbon sequestration into their sediment beds, but at the same time they are affected by accelerating sea level rise induced by climate warming. Consequently, an important question is how organic carbon accumulation rates (OCAR) of salt marshes will respond to future accelerating rates of relative sea level rise (RSLR). To date, existing insights are either based on (1) comparison of geographically distant marsh sites, differing in local RSLR rates but also in other environmental conditions that additionally can affect OCAR, or (2) experiments in given marsh sites, in which proxies for RSLR are manipulated, but run over periods of years instead of decades, the latter being the relevant time scale of marsh responses to RSLR. Here we bridge these shortcomings by studying the OCAR over four decades at two nearby salt marsh sites in the Netherlands, with similar environmental conditions, but with one site experiencing an accelerated RSLR rate of 9.7-11.7 mm yr (i.e., within the range of projected global mean sea level rise rates by 2100) due to local land subsidence induced by gas extraction, while the other site does not experience subsidence and has a low background RSLR rate of 2.0 mm yr (i.e. close to the current global mean sea level rise rate). Our results reveal that the salt marsh site experiencing the accelerated RSLR rates shows OCAR values that are on average twice as high as those found in the marsh site experiencing the low background RSLR rates. Moreover, the increase of OCAR in response to faster RSLR was even more pronounced (i.e. 63 % increase) on marsh levees within 10 m from tidal creeks, while this was more subtle (i.e. 27 % increase) in marsh basins at a distance of 30-40 m from the creeks. These observations of increased OCAR are mainly attributed to increased sediment accretion rates (SAR) in response to (1) increased tidal inundation due to accelerated RSLR and (2) larger sediment supply due to closer proximity to creeks, while sediment organic carbon content was relatively little affected. Our findings support expectations that nature-based climate mitigation actions, through salt marsh conservation and restoration, are sustainable on the long term of the coming decades, and are even likely to become more effective with future accelerations in global sea level rise, at least for macrotidal sites not limited by sediment supply.