Impacts of Holocene and modern sea‐level changes on estuarine mangroves from northeastern Brazil

Abstract

Projections of the impacts of modern Relative Sea Level (RSL) rise on estuarine mangroves should be supported by coastal topographic data and records of mangrove dynamics under past RSL change. This work identified inland and seaward mangrove migrations along the Jucuruçu River (Bahia, Northeastern Brazil), during the Holocene based on sedimentary features, palynological and geochemical (δ 13 C, δ 15 N, C/N) data integrated with digital elevation models. During the Middle Holocene, in response to RSL rise, the estuary saw mangrove forest establish up to~37 km inland. RSL stood between -1.4 (+0.36/-2.2 m) and +1 (2.19/0.2 m) around 7400 cal yr BP, and rose to a highest position of +3.25 (4.22/2.45 m) reached around 5350 cal yr BP. That marine incursion caused the inland replacement of freshwater vegetation by mangroves on tidal flats. Since then, the estuary experienced RSL fall, reducing inland tidal water salinity towards the Late Holocene, making that the mangroves were replaced by freshwater floodplain vegetation. Today, in the seaward part of the estuary near its mouth, mangroves occupy an area of~10 km 2 along tidal channels. Considering a RSL rise of 98 cm up to the end of the 21 st century, at a rate significantly higher than that of Middle Holocene RSL rise (1.5 mm/yr) and fall (0.6 mm/yr), the current mangrove substrates are expected to drown and/or eroded near the coast, while new mangroves may establish inland, at topographically higher tidal flats in nowadays freshwater-tidal zones. Mangrove area could expand over 13 km 2 of coastal and flood plain. Following the same interaction between RSL/climate changes and Holocene mangrove dynamics, such upstream mangrove migration may be attenuated or intensified by changes in fluvial discharge.

Effects of the middle Holocene high sea‐level stand and climate on Amazonian mangroves

“…Determining the spatial and temporal scales of mangrove degradation are essential for assessing environmental disturbances and their drivers (Vogt et al ., 2014). Allogenic processes (Cecil, 2013), related to large‐scale disturbances in mangroves, are mainly attributed to RSL and climate changes (Cohen et al ., 2012, 2020a,b; Rodrigues et al ., 2021). By contrast, autogenic disturbances on mangroves occur on small time scales (annual to interannual, 0.5–10 years) and with local impacts on the ecosystem (Pickett and White, 2005) such as strong winds (Jeltsch, 1992), fires (Ratz, 1995) and insect attacks (Coley and Barone, 1996; Menezes and Mehlig, 2005).…”

“…In the present study, a geodesic landmark and the upper tidal flats, occupied by modern Bragança mangroves (2.1 ± 0.1 m amsl), were used as a reference to record RSL changes in the studied cores. Isotopic (δ 13 C and δ 15 N) and C/N analyses of sedimentary organic matter associated with mangrove pollen grains along topographically referenced cores have been used to identify trends of RSL changes (Cohen et al, 2005b(Cohen et al, , 2012(Cohen et al, , 2014a(Cohen et al, , 2015(Cohen et al, , 2020aLamb et al, 2006;Fontes et al, 2017;Ribeiro et al, 2018). The relationship between δ 13 C and C/N, as well as δ 15 N, allows for the identification of the source of sedimentary organic matter, mainly between end-members: terrestrial-and aquatic-derived organic matter (Deines, 1980;Schidlowski et al, 1983;Meyers, 1994;Tyson, 1995).…”

“…The transition from RSL rise to fall triggered several feedback mechanisms related to estuarine salinity gradients, tidal range, and input and vertical accretion of sediments on tidal flats (Cohen et al, 2020a). Depending on the interaction of these factors, mangroves may be degraded, expanded, or reestablished at a lower topographic level (Schwartz, 1965;Cohen et al, 2012Cohen et al, , 2014aCohen et al, ,b, 2018Woodroffe and Murray-Wallace, 2012;Ellisson, 2016).…”

“…Sea‐level changes have been the main mechanism driving mangrove spatial dynamics on the tropical Brazilian coast during the Holocene (Pessenda et al ., 2012; Cohen et al ., 2014a, 2020a; França et al ., 2015; Fontes et al ., 2017). These forests are very susceptible to variations in tidal flooding frequency (Blasco et al ., 1996; Fromard et al ., 2004; do Amaral et al ., 2006; McLeod and Salm, 2006; Cohen et al ., 2012; Krauss et al ., 2014; Alongi, 2015; Yao and Liu, 2017).…”

“…These forests are very susceptible to variations in tidal flooding frequency (Blasco et al ., 1996; Fromard et al ., 2004; do Amaral et al ., 2006; McLeod and Salm, 2006; Cohen et al ., 2012; Krauss et al ., 2014; Alongi, 2015; Yao and Liu, 2017). The highest relative sea level (RSL) in the Holocene [1–5 m above the modern sea‐level (amsl)] occurred at ~5500 cal a bp along the south‐eastern and north‐eastern Brazilian coast (Martin et al ., 2003; Angulo et al ., 2006; Suguio et al ., 2013; Lorente et al ., 2014; Boski et al ., 2015; Cohen et al ., 2020a; Toniolo et al ., 2020), and caused landward migration of mangroves into embayments, estuaries and lagoons (Castro et al ., 2013; Lorente et al ., 2014; França et al ., 2015; Rossetti et al ., 2015; Cohen et al ., 2014a,b, 2020b). RSL on the Rio Grande do Norte coast in north‐east Brazil (Fig.…”