Carbon burial records during the last ~40,000 years in sediments of the Liaohe Delta wetland, China

Abstract

Delta wetland sediments constitute a long-term natural carbon sink and play a critical role in the global carbon cycle. In this study, borehole core ZK3 (36.7-m long), drilled in 2012 in the Liaohe Delta wetland, was investigated to assess the rate of carbon sequestration and the factors influencing carbon burial since the Late Pleistocene. Here we report the results of integrated analyses of the core, including its sedimentary lithology, grain size, foraminiferal abundance, chemical elements, and accelerator mass spectrometry (AMS)¹⁴C and optically stimulated luminescence (OSL) dates. The sedimentary environment has evolved from a fluvial-deposit, limnetic-deposit, littoral-deposit, shallow sea–deposit, and finally to a delta-deposit environment since 40,000 cal yr BP. Environmentally induced differences in apparent mass accumulation rates (AMARs) of organic carbon (OC) have been significant; they have ranged between 3.73 and 30.77 g/(m²·yr). The fact that the highest rates were associated with the delta-deposit environment indicates that the rate of carbon sequestration was greater in the sediments of estuarine wetlands than sediments of the continental shelf. The chemical index of alteration (CIA) proxy responded to several cold events, and there was a positive correlation between the CIA and OC-AMAR. Climate may therefore have regulated the excursions of ecosystem productions and in turn impacted the dynamics of carbon burial.

Introduction

With the increasing global average atmospheric carbon dioxide (CO₂) concentration, a single emission–reduction strategy has been switched to a plan that combines reducing anthropogenic emissions of CO₂ (mitigation) with strengthening carbon (C) storage of natural ecosystems because of their high C sequestration rates and capacity for carbon storage (Canadell and Raupach, 2008). As an important part of blue carbon sinks, coastal wetlands play a critical role in carbon sequestration and mitigation of global warming effects (Bridgham et al., 2006b; Chmura et al., 2003; DeLaune and White, 2012; Mcleod et al., 2011). The organic carbon pool in sediments may exceed the amount of carbon in living vegetation by a factor of 2–3 (Lettens et al., 2005; Schlesinger, 1990). Sediment carbon sequestration is believed to be one of the cost-effective ways to mitigate the adverse effects of atmospheric CO₂ increases, including global warming (Naizheng et al., 2011; Olson, 2013; Sommer and Bossio, 2014).

Costal wetland sediments constitute a long-term natural carbon sink and thus affect atmospheric CO₂ concentrations (Kroeger et al., 2017). High rates of C accumulation in coastal sediments are associated with high plant productivity and burial of organic matter through sedimentation. Substantial spatial variability of wetland C accumulation rates has been related to differences in tidal range, elevation, freshwater input, and sediment availability (Craft and Richardson, 1998; Kirwan and Guntenspergen, 2010; Unger et al., 2016). The two species of carbon in the environment are organic carbon (OC) and inorganic carbon (IC) (Koziorowska et al., 2017, 2018). The material flux reaching costal wetland sediments comprises a mixture of these two forms. Whether they are autochthonous or allochthonous, both OC and IC are readily deposited in bottom sediments (Teske et al., 2011). The former are intensively mineralized in the water column and at the sediment—water interface (Holding et al., 2017; Teske et al., 2011), where the redox conditions determine the efficiency of mineralization and biogenic element exchange between sediments and the overlying water (Ingall et al., 2005; Jørgensen et al., 2005).

Deltaic coastal wetlands experience vegetation and habitat shifts associated with changes in relative sea level, river flooding, channel migration, and storm deposition or erosion. Therefore, rates of C accumulation will change over time concomitant with changes in sedimentation, hydrodynamics, freshwater availability, and productivity. Within the sediment profile, loss of OC can occur through microbial respiration and erosion and tidal export, whereas storm deposition and inputs of fresh organic matter will lead to increases (DeLaune et al., 2018). The burial of organic matter during the postglacial period is linked with paleoenvironmental changes (Yang et al., 2011). Quantitative analyses of carbon burial records have involved the determination of apparent mass accumulation rates, which have helped to clarify stratigraphic divisions, processes associated with evolution of the paleoenvironment, and the associated timespans. Quantification of C accumulation rates may depend on sediment compaction over time and consequently require a bulk density correction to validate comparisons between depths and among different systems. Sediment accumulation rate, which is inversely correlated with % OC, is a major factor controlling the organic carbon flux to sediments (Cui et al., 2016; Ramirez et al., 2016; Zhao et al., 2015) (Cui et al., 2016; Ramirez et al., 2016; Zhao et al., 2015).

The goal of this study was to provide quantitative information about long-term C burial in the Liaohe Delta (LHD), the northernmost coastal wetland in China. Combined with core elevation and sea level change data, sedimentary lithology, grain size, radiocarbon dating and foraminiferal abundance were analyzed to establish the chronostratiographic framework of the LHD since ~40,000 year BP of the late Pleistocene. Carbon burial and the factors that influence carbon burial are discussed in the context of this information.