Open-Ocean Minima in δ13C Values of Particulate Organic Carbon in the Lower Euphotic Zone

Abstract

Extensive studies in the 1980s-1990s led to the characterization of latitudinal variations in sea surface δ 13 C values of particulate organic carbon (δ 13 C POC), and relationships were found with CO 2 concentrations, temperature, growth rates, and cell geometries. Surprisingly, no large-scale efforts have been made to describe variations in δ 13 C POC values over depth in the water column. Here we compile published examples demonstrating a widespread isotopic pattern in particulate organic carbon (POC) of the upper water column. In 51 vertical profiles, δ 13 C POC values in the lower euphotic zone on average are 1.4 lower than δ 13 C POC values in the upper euphotic zone of open ocean settings. In a majority of locations this vertical decrease in δ 13 C POC values is >2 and up to 5 , larger than the commonly recognized vertical δ 13 C variation in dissolved inorganic carbon over the same depths. We briefly review hypotheses and supporting evidence offered by previous studies of individual water columns: The observed patterns could result from vertical differences in photosynthetic growth rates or community composition, biochemical composition of organic matter due to degradation, isotopic disequilibrium within the dissolved inorganic carbon pool, particle dynamics, or seasonal vertical mixing. Coordinated isotopic, biological, and seawater chemistry data are sparse, and consistent drivers of this widespread isotopic pattern are currently elusive. Further work is needed to adequately characterize the environmental conditions coinciding with this pattern, to test its origins, and to determine if the magnitude of upper water column δ 13 C POC variations could be a useful marker of upper ocean carbon cycle dynamics.

The OMZ and Its Influence on POC in the Tropical Western Pacific Ocean: Based on the Survey in March 2018

“…The vertical distribution of POC is closely related to the biogeochemical processes at each water depth. Photosynthesis is the main source of POC in the upper waters of the ocean (Song, 2010;Close and Henderson, 2020). In this study, POC and Chla concentrations showed a significant positive correlation in the water column of 0-300 m (r 0.339, p < 0.01, n 139), which further indicated the close relationship between POC and phytoplankton in the upper waters of the Kocebu seamount area.…”

“…In the water column of 500-1,000 m, although the POC continued to decrease, its decomposition rate was significantly lower than that in the water column of 150-300m. The reason for this phenomenon may be that the easily degradable POC was decomposed in the shallower water column, whereas the remaining POC in the deeper water column was difficult to decompose (Close and Henderson, 2020;Weber and Bianchi, 2020). In addition, it may also be caused by a significant decrease in the seawater temperature and DO.…”

“…In a study on the vertical distribution of δ 13 C-POC in the eastern tropical North Pacific, Jeffrey et al (1983) also reported a consistent conclusion with this study, that is, δ 13 C-POC was relatively uniform and heavier in the upper waters, and it became lighter gradually and remained stable in the deep waters. Based on the analysis of δ 13 C-POC in 51 profiles in the global ocean, Close and Henderson (2020) reported that δ 13 C-POC had the abovementioned vertical distribution rule in most regions. Microorganisms first decompose the heavier δ 13 C components in the POC, such as amino acids and carbohydrates, resulting in lighter δ13C components, and this fractionation process of δ13C-POC is one of the main reasons for the lighter δ13C-POC with the increase in water depth (Close and Henderson, 2020;Kharbush et al, 2020).…”

“…Based on the analysis of δ 13 C-POC in 51 profiles in the global ocean, Close and Henderson (2020) reported that δ 13 C-POC had the abovementioned vertical distribution rule in most regions. Microorganisms first decompose the heavier δ 13 C components in the POC, such as amino acids and carbohydrates, resulting in lighter δ13C components, and this fractionation process of δ13C-POC is one of the main reasons for the lighter δ13C-POC with the increase in water depth (Close and Henderson, 2020;Kharbush et al, 2020). These results indicate that δ 13 C-POC could be an important reference for the study of POC decomposition.…”

Description of a global marine particulate organic carbon-13 isotope data set

“…A recent collection of 303 measurements of δ 13 C POC has been provided by Close and Henderson (2020), largely based on data gathered from individual publications referenced therein. Since our analyses originally relied on data sources that differed from those of Close and Henderson (2020), we find our collection to be as yet incomplete. Especially measurements from national databases might provide a huge future benefit.…”