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Title: Biogeochemical cycling and environmental footprints of peatland-derived organic carbon in the coastal ocean in Southeast Asia
Authors: Zhou, Yongli
Keywords: Science::Geology
Issue Date: 2021
Publisher: Nanyang Technological University
Source: Zhou, Y. (2021). Biogeochemical cycling and environmental footprints of peatland-derived organic carbon in the coastal ocean in Southeast Asia. Doctoral thesis, Nanyang Technological University, Singapore.
Project: MSRDP-P32
RG 175/16
Abstract: Each year, 0.2–0.25 Pg C of dissolved organic carbon (DOC) is delivered from land to ocean by riverine transport. This riverine flux of terrigenous DOC alone is sufficient to support the turnover of the entire oceanic DOC pool, which means that terrigenous DOC can in theory dominate the DOC pool in the ocean. However, data on the chemical composition of DOC in the ocean have generally been interpreted as showing that only a small fraction (<25%) of the oceanic DOC pool is of terrestrial origin. This discrepancy opens an important question for our understanding of the global carbon cycle: where has the “missing” terrigenous DOC gone? Previous studies have reported that a large proportion of the terrigenous DOC can be remineralized into CO2 in some of the major shelf seas, which might explain the low abundance of terrigenous DOC in the deep ocean. However, the extent, rates, and pathways of the biogeochemical processing of terrigenous DOC in the shelf seas are still far from clear, which hampers our understanding of how much terrigenous DOC accumulates in the ocean. In addition, studies of the biogeochemical cycling of terrigenous DOC have so far mainly focused on latitudes outside of the tropics, which results in very limited knowledge about the terrigenous DOC cycling in tropical seas. Southeast Asia is home to the largest area of tropical peatlands and is a hotspot of terrigenous DOC export to the coastal ocean. The riverine flux of terrigenous DOC in Southeast Asia amounts to 21 Tg C yr 1, accounting for 10% of the global flux. The majority of this terrigenous DOC flux enters the Sunda Shelf Sea, where the terrigenous DOC has a relatively long residence time of 1–2 years. Here, we studied the composition, degradation, and environmental footprints of the terrigenous DOC derived from Southeast Asian peatlands in the rivers, estuaries, and the Sunda Shelf Sea. Fluorescent dissolved organic matter in 225 water samples from peat-draining rivers, estuaries, and coastal waters of Sarawak, Borneo were analyzed with parallel factor analysis, which resolved the composition of the peatland-derived DOC. The terrestrial humic like components all showed conservative mixing with seawater, indicating that the peatland-derived DOC is exported to the shelf sea with little biogeochemical processing in the rivers. Using the terrestrial humic-like component as a quantitative tDOC tracer, we further estimated that 20–25% of the DOC in coastal waters of Sarawak is of terrestrial origin. To study the biogeochemical fate of terrigenous DOC in the shelf sea, biweekly to monthly water samples and autonomous sensor data were collected from the Singapore Strait, which is close to the main peatlands on Sumatra. Terrigenous DOC derived from peatlands is advected from the east coast of Sumatra to the Singapore Strait during the Southwest Monsoon. We analyzed the DOC concentration, the seawater carbonate system and stable carbon isotope composition using an isotope mass balance approach and found that 60–70% of the terrigenous DOC from peatlands is remineralized in the coastal waters of the Sunda Shelf Sea. Based on results from degradation experiments, we further infer that in total 70–80% of the terrigenous DOC is likely remineralized within the shelf and the remaining 20–30% is likely exported to the open Indian Ocean. Approximately 50% of the remineralized DOC remains in the shelf water as dissolved inorganic carbon and causes a decline in pH of up to 0.10 in the Singapore Strait during the Southwest Monsoon. Most of the remineralized DOC is likely to be lost to the atmosphere eventually, fueling a CO2 efflux of 2.4–4.9 mol m 2 yr 1. Our results indicate that a significant proportion of the terrigenous DOC is remineralized in the Sunda Shelf Sea before reaching the open Indian Ocean, and causes pronounced CO2 emissions and ocean acidification. Photodegradation has been recognized as an important pathway of terrigenous DOC processing in natural waters. We therefore further quantified the contribution of photodegradation to the remineralization of terrigenous DOC in the Sunda Shelf Sea. We quantified the photochemical efficiency (apparent quantum yield) of the terrigenous DOC and modeled the photodegradation process. Our modeling results show that 18–26% of the terrigenous DOC can be directly remineralized by natural solar radiation, accounting for a moderate but significant proportion of the total quantity of the remineralization. We indicate that photodegradation is an important process for the removal of terrigenous DOC in the Sunda Shelf Sea and that tropical shelf seas are likely stronger sources of photo-produced CO2 to the atmosphere compared to the temperate and the Arctic shelf seas. However, our results also indicate that photodegradation alone cannot account for the observed remineralization of tDOC. We suggest that photo-enhanced bio-remineralization likely plays an important role in the tDOC remineralization and needs further research.
DOI: 10.32657/10356/153682
DOI (Related Dataset):
Rights: This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).
Fulltext Permission: open
Fulltext Availability: With Fulltext
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