Variations in ¹⁴C Reservoir Ages of Black Sea Waters and Sedimentary Organic Carbon During Anoxic Periods: Influence of Photosynthetic Versus Chemoautotrophic Production

Abstract

ABSTRACT. Radiocarbon activity of dissolved inorganic carbon has been measured in the northwestern Black Sea. Both continental shelf and open-sea profiles show that surface waters are in equilibrium with the atmosphere. The observed distribution of 14 C activity shows a weak contribution of the deep 14 C-depleted CO 2 to the photic zone. Such a distribution of 14 C within the water column is unable to explain the aging of sedimentary organic matter and reservoir ages greater than 500 yr. A contribution of production by chemoautotrophic bacteria feeding on 14 C-depleted methane at the boundary of the oxic and anoxic zones is a realistic hypothesis. Also, a contribution to sedimentary organic carbon estimated at <15% of the photosynthetic primary production could explain 14 C reservoir ages greater than 1300 yr.

Implications for chloro- and pheopigment synthesis and preservation from combined compound-specific δ&lt;sup&gt;13&lt;/sup&gt;C, δ&lt;sup&gt;15&lt;/sup&gt;N, and Δ&lt;sup&gt;14&lt;/sup&gt;C analysis

“…Due to this instability of primary pigments, the 14 C concentrations of pigments deposited in surface sediments would be expected to mirror recent surface water DIC 14 C concentrations. Instead, 14 C concentrations of chlorophyll-a (−183.2 ± 15.3‰) and pheophytin-a (−127.9±11.9‰ and −109.3±11.6‰) at stations P177 and P128, respectively, are much lower than DIC 14 C values represented by co-occurring bivalves from 17 m (56.1 ± 3.3‰) to 96 m water depth (9.9 ± 3.8‰) and previously reported values (Jones and Gagnon, 1994;Fontugne et al, 2009), translating into ages of 570 to 1260 cal yrs BP (Table 1). Potential explanations for the low 14 C concentrations include utilization of different DIC pools, protective mechanisms associated with sedimentary matrices or faecal pellets, and contribution of pre-aged terrestrial pigments.…”

“…Equation 1is numerically integrated from year 1810 to 2010 and evaluated at 20 m depth since highest chlorophyll-a concentrations have been found between 10 m and 30 m water depth (Krupatkina et al, 1991;Repeta and Simpson, 1991;Chu et al, 2005). The solution is fitted to scattered observations (Ostlund and Dryssen, 1986;Jones and Gagnon, 1994;Siani et al, 2000;Fontugne et al, 2009; this study) using K and µ as fit coefficients. Values of K and µ are about 7 × 10 3 m 2 yr −1 (∼2 cm 2 s −1 ) and…”

Using Stable Carbon Isotopes to Quantify Radiocarbon Reservoir Age Offsets in the Coastal Black Sea

“…Oxidation of old methane could be another explanation, as it seems to impact on the 14 C composition of DIC of the present day Black Sea waters in the deeper part of the basin (slope and deep basin) (Fontugne et al, 2009). However, for very negative δ 13 C values typical of methane oxidation (e.g.…”

Geomicrobiology of the carbon, nitrogen and sulphur cycles in Powell Lake: a permanently stratified water column containing ancient seawater

“…The Black Sea, which is the Earth's largest contemporary meromictic water column, may be most similar to Powell Lake with respect to long-term stratification. It was established as a marine basin by connection to the Mediterranean Sea at approximately the same time as Powell Lake was disconnected from the ocean (Yanko-Hombach et al, 2014), but the 'age' of Black Sea deep waters is estimated to be <1000 years (Fontugne et al, 2009;Soulet et al, 2019).…”