A novel bacteriohopanepolyol biomarker reveals anaerobic ammonium oxidation has been an important process in Gulf of Alaska nitrogen cycling over the last ~60 ka.

Abstract:

Anaerobic ammonium oxidation (anammox) is one of the major sinks of bio-available nitrogen from the marine system. Anammox is estimated be responsible for ca. 30% of dinitrogen gas produced in modern open oceans [1]. Tracing the anammox process in the past has been temporally-limited by the lability of the lipid biomarkers synthesised by anammox bacteria. These ladderane lipids are highly susceptible to degradation [2,3], and are not well preserved in the sedimentary record [4]. Recently, however, bacteriohopanetetrol stereoisomer (BHT isomer) was identified as a more recalcitrant biomarker for pelagic anammox, expanding the potential to study past anammox activity [5].

Here, we investigate BHT isomer concentration (Fig. 1a) and BHT isomer ratio (Fig. 1b) in sediments from the last ~60 ka in the Gulf of Alaska (IODP Expedition 341, Site U1419) [6]. Our data show increased deposition of pelagic anammox bacteria, paralleled by inorganic proxy records for oxygen-depleted bottom water conditions (total sulfur and pyrite-bound iron to highly reactive iron ratio; Fig. 1, c,d). This indicates that the oxygen minimum zone in the Gulf of Alaska likely fluctuated following past climate changes.

[1] Ward, B.B. 2013. Science 341, 352-353.

[2] Sinninghe Damsté, et al. 2002. Nature 419, 708-712.

[3] Rush et al. 2011. Geochimica et Cosmochimica Acta 75, 1662-1671.

[4] Jaeschke et al. 2009. Paleoceanography 24, PA2202, doi:10.1029/2008PA001712.

[5] Rush et al. 2014. Geochimica et Cosmochimica Acta 140, 50-64.

[6] Jaeger et al., 2015. Proceedings of the Integrated Ocean Drilling Program, Volume 341

[7] Sáenz et al., 2011. Organic Geochemistry 42, 1351-1362.

Figure 1. Anammox in the Gulf of ­­Alaska over ~60 ka. (a) bacteriohopanetetrol and bacteriohopanetetrol stereoisomer (BHT isomer) concentrations, (b) BHT isomer ratio with known ranges for sediment and anammox cultures [5,7], (c) S (% weight), and (d) pyrite-Fe over highly reactive Fe.