Low carbon accumulation in a macro-tidal mangrove forest on the Amazon coast

The important role mangrove forests play in sequestering organic carbon is well known, yet rates of organic carbon accumulation in macro-tidal mangrove ecosystems are poorly resolved. Here we use ²¹⁰Pb dating to present a 125-yr record of carbon, nutrient and trace metal accumulation in sediments from an Amazon macro-tidal mangrove forest. We find that the rate of organic carbon accumulation ranged from 23.7 to 74.7 g m⁻² yr⁻¹ (average 38 ± 13.5 g m⁻² yr⁻¹), significantly lower than global averages for mangrove forests. These low rates may be associated with sediment grain-size and sediment–water interface processes that drive organic matter oxidation and reduce carbon stocks in these highly dynamic macro-tidal forests. Total nitrogen accumulation ranged from 1.4 to 5.1 g m⁻² yr⁻¹ (average 2.7 ± 0.9 g m⁻² yr⁻¹) and phosphorus from 1.5 to 8.4 g m⁻² yr⁻¹(average 4.3 ± 1.9 g m⁻² yr⁻¹). Trace metal accumulation rates (As, Pb, Cr, Cu, Mn, Ni, Zn, Hg, Bo, V, Co, Mo, S, and Ba) were also lower than other tropical mangrove forests globally, but trace metal in more recent sediments for Mn, As, Cu, and Hg were elevated, likely reflecting human footprint in the region since early the 20^th century. The ability to accurately quantify carbon accumulation rates in mangrove ecosystems is critical for climate change mitigation strategies and the implementation of carbon offset schemes globally.