Distinct oxygenation modes of the Gulf of Oman during the past 43,000 years – a multi-proxy approach

Abstract

Abstract. Climatic conditions and its change shape the strength and extent of the oxygen minimum zone (OMZ). The presence and variability of the OMZ in the Arabian Sea is of importance for their ecosystem. The state of oxygenation has, for instance, an impact on the pelagic and benthic faunal community or the nitrogen and carbon cycles. The understanding of the dynamics of the OMZ, its marine environmental is of importance due to its climate feedbacks. In this study, we combined three independent proxies to reconstruct the oxygenation state of the water column and bottom water in the Gulf of Oman for the past about 43 ka for the first time. We used nitrogen isotopes (δ15N) as well as the alkane ratio (lycopane + n-C35)/n-C31 and benthic foraminiferal faunal analysis to reconstruct the strength of the OMZ in the water column and bottom water oxygenation, respectively. Our results show that the Gulf of Oman experienced strong pronounced OMZ and bottom water deoxygenation during the Holocene. Contrary, during the Last Glacial Maximum (LGM)/ Marine Isotope Stage (MIS) 2 the Gulf of Oman was very well ventilated with a highly diverse benthic foraminiferal community. This may have been caused by stronger wind-induced mixing and better ventilation by oxygen-rich water masses. Our results also show moderate oxygenation during MIS 3 with deoxygenation events during most of the warmer Dansgaard-Oeschger (D/O) events. We propose two distinct oxygenation modes for the Gulf of Oman: 1) a stable period of either strongly pronounced water column OMZ and bottom water deoxygenation or well-oxygenated water column and bottom water conditions and 2) an unstable period of oscillating oxygenation states between moderately oxygenated (stadials) and deoxygenated (interstadial, D/O events) conditions.

Spatial and temporal variability of sea surface temperatures and monsoon dynamics in the northwestern Arabian Sea during the last 43 kyr

“…Notably, reconstructed strong OMZ during D-O 10 and 4 events at the core site (Figure 4f, Burdanowitz et al, 2024) are in line with somewhat lower SST. We attribute this to an enhanced influence of the SW monsoon winds and/or more northward-extended influence of the Oman upwelling area at the core site.…”

“…However, our record exhibits a distinct cold signal during D-O 2, which is even lower comparable to H2, and a subsequent SST increase. Burdanowitz et al (2024) noted a less pronounced OMZ in the water column but sub-/anoxic conditions in the bottom water at the core site. They attributed this to an intensified inflow of oxygen depleted RSW at intermediate depths and/or weak inflow of Antarctic Intermediate Water into the Gulf of Oman.…”

“…The analysis of oxygenation levels in both the water column (Figure 4f) and bottom water (Figure 4g) from the same core reveals different impacts of the H4 event (Burdanowitz et al, 2024). While the observed increase in water column oxygenation corresponds with the decline in SST, the changes in bottom water oxygenation are less marked.…”

“…The piston core SL167 (741 cm long) was collected in the northeastern part offshore the Oman margin in the Gulf of Oman in the northwestern Arabian Sea (22° 37.15′ N, 059° 41.49′ E; 774 m water depth) during RV METEOR cruise 74/1b in September 2007 (Bohrmann et al, 2010). The age model of SL167 is based on twenty-one radiocarbon dates of surfacedwelling planktonic foraminifera and is published in Burdanowitz et al (2024). The core includes the time period from about 3 to 43 ka.…”

“…Alkenones were measured containing 2 cm of sediment for the upper 162 cm in 2 cm and 4 cm of sediment below 162 cm, due to lower organic content (<1.5 %). To obtain total lipid extract (TLE) about 3 to 18g sediment were extracted by a Dionex Accelerated Solvent Extractor (ASE 200) using dichloromethane (DCM) and methanol (MeOH) (ratio 9:1) as solvent as described in Burdanowitz et al (2024).Temperature and pressure were kept constant at 100 °C and 1000 PSI for five minutes. This procedure was performed three times.…”