Late Holocene mangrove dynamics dominated by autogenic processes

Abstract

Generally, palaeoenvironmental interpretations consider only allogenic processes, when autogenic factors may have a strong influence on proxies of stratigraphic sequences. For instance, the Holocene history of the vegetation along the southern littoral of the State of Bahia in north-eastern Brazil is characterized by mangrove dynamics controlled by allogenic processes. However, over smaller timescales (~700 years), autogenic processes may have controlled vegetation dynamics and hence observed pollen distribution. This work proposes tidal channel dynamics as one of the main cause for changes in pollen assemblage along the studied stratigraphic profiles during the last centuries, based on sedimentology, pollen and elemental analysis (δ 13 C, δ 15 N and C/N) and radiocarbon dating of sedimentary organic matter from two cores sampled from an abandoned meander and a tidal flat at the mouth of the Jucuruçu River. One core was sampled from a mangrove formed during the past~550 cal yr BP. Another core recorded sediments in a várzea forest (swamp seasonally and permanently inundated by freshwater) located~2.7 km from the current shoreline, which displayed a maximum age of~680 cal yr BP. Two facies associations were identified: tidal channel (A) and tidal flat/oxbow lake (B). This work proposes allogenic processes as the main driving forces controlling the wetlands dynamics at the studied site during the Holocene. However, our data also reveal that part of the changes in vegetation over the last~700 years reflect tidal channels and tidal flats development, which represent autogenic processes. The change in timescale analysis from the Holocene to recent centuries may have weakened the influence of allogenic factors. However, this needs interpretation with reference to the spatial scale of the depositional environment as the larger the depositional system analyzed, the stronger the influence of autogenic processes on stratigraphic sequences over longer timescales.

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

“…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). The tidal channel dynamics related to erosion and sediment deposition on tidal flats can also affect the spatial distribution of mangroves locally with some effect on the pollen signal down‐core (Moraes et al ., 2017; Ribeiro et al ., 2018). These processes are inherent to the depositional system and vegetation dynamics (Beebower, 1964; Cohen et al ., 2020a).…”

“…The isotopic and elemental characteristics of sedimentary organic matter accumulated on the studied tidal flats depend on the in situ vegetation and contribution of fluvial and marine organic matter (Dittmar et al, 2001(Dittmar et al, , 2006. Depending on sedimentation rates and core depth, a stratigraphic profile can record events in different time scales: for example, evidence of daily tides through heterolithic bedding (Daidu et al, 2013), to decadal/secular/millennial scales (Cohen et al, 2015;França et al, 2016;Moraes et al, 2017;França et al, 2019). In the studied cores, oscillations of isotopes, C/N values and pollen percentages are observed from centimeter to meter scales.…”

Allogenic and autogenic effects on mangrove dynamics from the Ceará Mirim River, north‐eastern Brazil, during the middle and late Holocene

“…An example of autogenic process controlling the palaeoflora was presented by Moraes et al . (), where the authors proposes allogenic process as the main driving forces controlling the wetlands dynamics along the Jucuruçu River‐Bahia, north‐eastern Brazil during the Holocene. However, his work also reveals that part of the changes in vegetation over the last ~700 years reflects tidal channels and tidal flats development, which represent autogenic process.…”

Reconstitution of Atlantic Rainforest vegetation dynamics since the Late Pleistocene at southeastern (Espirito Santo state coast and Minas Gerais eastern) and northeastern (Bahia southern) Brazil

“…Recently, some papers based on palynology from Southern Bahia were published (FONTES et al, 2017;MORAES et al, 2017), demonstrating mangrove retraction and variations in the geological landscape since ~7,500 cal yr BP, but this is the first research from extreme Southern Bahia to test the hypothesis that forests in this centre of endemism have been stable through the Holocene.…”

“…Remote sensing data obtained by Landsat optical images (MORAES et al, 2017) indicate that the plateau, which is now dominated by pasture, was originally covered by Atlantic Rainforest; while floodplains are dominated by herbaceous vegetation. Gallery forests grow along the scarps, while mangroves form a fringe along tidal channels under marine influence.…”