Sea level rise sedimentary record and organic carbon fluxes in a low-lying tropical coastal ecosystem

Highlights

• Tropical saltmarsh sediment cores provide useful information on local sea level rise.

• A sediment core showed signals of increased marine influence due to sea level rise.

• The maximum sediment accretion rate was 2.9 mm yr^− 1 and acceleration was 0.22 mm yr^− 2.

• By 2100 mean sea level could be between 14 and 86 cm above the present height.

• Organic carbon flux ranged from 7 to 73 g m^− 2 yr^− 1 and carbon stock was 10.1 Mg ha^− 1.

Abstract

Sea level rise, one of the most evident effects of recent climate change, is already impacting coastal ecosystems. Because of its low relief, population increase and economic development during the last century, the Celestun lagoon is highly vulnerable to global change, including sea level rise. Here, we study a sediment core from a tropical saltmarsh to evaluate the impact of sea level rise on sediment accretion and carbon fluxes. Some geochemical indicators in a ²¹⁰Pb dated sediment core showed clear signals of marine influence. This was reflected on increased accretion rates, from 0.3 ± 0.1 mm yr^− 1 in ≈ 1941 to 2.9 ± 1.2 mm yr^− 1 in 2012. These accretion rates were similar to eustatic sea level rise, and mean acceleration ranged from 0.037–0.22 mm yr^− 2, implying a SLR by 2100 ranging from 14 to 86 cm. The one-century (1917–2013) organic carbon stock in sediments was calculated to be 10.1 ± 0.2 Mg C ha^− 1. During the same period, the organic carbon flux, corrected for organic carbon degradation, increased from 7 ± 3 g C m^− 2 yr^− 1 to 73 ± 26 g C m^− 2 yr^− 1, attributed to the impact of sea level rise in this ecosystem. This work shows that tropical saltmarshes may provide useful information on sea level rise, and the presented methodology may be used where instrumental records do not exist or they are too short.

Introduction

Sea level rise is threatening coastal ecosystems, reducing ecosystem services (e.g. storm protection, niche for species larval stage) and may, in the long term, cause profound socio-economic changes (SLR; Stocker et al., 2013). Low-lying coastal ecosystems are the most vulnerable ones to recent SLR. The Ría Celestún (Celestun coastal lagoon, Gulf of Mexico) is located in the northwestern Yucatan peninsula and encompasses a large diversity of ecosystems and species, some of which are endangered. It is inhabited by almost 7000 people, > 25% living in extreme poverty (SEDESOL, 2016). The main economic activities in the area are fishing, salt extraction and tourism (UNESCO, 2016). The maximum altitude of the Celestun Biosphere Reserve is only 3 m above local mean sea level, so it is highly vulnerable to SLR and extreme climatic events, such as tropical storms and hurricanes.

Although recent eustatic SLR, mainly caused by continental ice melting and seawater thermal expansion, is rather well known and thoroughly studied, specific ecosystem and socio-economic impacts will be caused by local SLR, i.e. eustatic SLR corrected for vertical movements of the earth's crust and by long-term changes of atmospheric pressure, ocean currents and temperature (e.g. Nicholls and Cazenave, 2010, Wu et al., 2010).Therefore, in order to propose scientifically sound adaptation and mitigation strategies, long-term sea level time series are needed. However, for most of the world's coastal regions these are inexistent, too short or too sparse to obtain sound information. Near the Celestun coastal lagoon (90 km NE), the Servicio Mareográfico Nacional (Universidad Nacional Autónoma de México) manages a tide gauge in Progreso de Castro (Yucatan State), which has provided a rather continuous time series of tidal elevation during the period 1946–1985, partial records during 1994, and the continuous record was reestablished in 2012 (SMN, 2017). For the period 1953–1992, the estimated SLR rate in Progreso is 2.5 ± 1.2 mm yr^− 1 (Zavala-Hidalgo et al., 2010). Recently, eustatic SLR rate is estimated to be 2.8 ± 0.8 mm yr^− 1 during the period 1993–2009 (Church and White, 2011).

In the absence of long term tide gauge records, the only option to reconstruct the SLR trends within the past century may be ²¹⁰Pb dated sediment cores from coastal ecosystems, under the assumption that, to be preserved, the accretion rates in these environments are at least equivalent to the SLR elevation. Tropical saltmarshes (locally known as marismas) are usually found along semi-sheltered low-energy coastlines, protected from the open ocean, located behind the mangrove fringe in the highest topographic position within the tidal range, so they are intermittently inundated by medium to high tides. High evaporation and relatively infrequent flooding favor the formation of hypersaline soils that can be colonized by halophyte “glassworts”, vegetation (i.e. Batis maritima and Salicornia pacifica) that can tolerate inundation with seawater and high soil salinity (Costa et al., 2009, Ruiz-Fernández et al., 2016) or remain non-vegetated. When sediment supply is sufficient, saltmarshes are able to keep pace with SLR (Nolte et al., 2013), so the sedimentary record can provide useful information on local SLR (e.g. Lynch et al., 1989, Parkinson et al., 1994, Sanders et al., 2010a, Ruiz-Fernández et al., 2016).

In this work, we study the geochemical signals of SLR in a ²¹⁰Pb dated sediment core collected from a tropical saltmarsh in Celestun, a low-lying coastal lagoon in the southern Gulf of Mexico, and compare sediment accretion rates with eustatic SLR and with a nearby tide gauge station. It is expected that this information will help to better protect and plan sound adaptation and mitigation strategies for this important protected area.