Deoxygenation dynamics above the western Nile deep-sea fan during sapropel S1 at seasonal to millennial time-scales

Abstract

Abstract. Ocean deoxygenation is a rising threat to marine ecosystems and food resources under present climate warming conditions. Organic-rich sapropel layers deposited in the Mediterranean Sea provide a natural laboratory to study the processes that have controlled the changes in seawater oxygen levels in the recent geological past. Our study is based on three sediment cores spanning the last 10 thousand years (10 kyr BP) and located on a bathymetric transect offshore the western distributaries of the Nile delta. These cores are partly to continuously laminated in the sections recording sapropel S1, which is indicative of bottom-water anoxia above the western Nile deep-sea fan. We used a combination of microfacies analyses and inorganic and organic geochemical measurements to reconstruct changes in oxygenation conditions at seasonal to millennial time-scales. The regular alternations of detrital, biogenic and chemogenic sublayers in the laminated sequences are interpreted in terms of seasonal changes. Our microfacies analyses reveal distinct summer floods and subsequent plankton blooms preceding the deposition of inorganic carbonates formed in the water-column during spring-early summer. The isotopic signature of these carbonates suggests year-round anoxic to euxinic bottom waters resulting in high levels of anaerobic remineralisation of organic matter and highlights their potential to reconstruct seawater chemistry at times when benthic fauna was absent. Synchronous changes in terrigenous input, primary productivity and past oxygenation dynamics on millennial time-scales obtained by our multi-proxy study show that runoff-driven eutrophication played a central role in driving rapid changes in oxygenation state of the entire Levantine Basin. Rapid fluctuations of oxygenation conditions in the upper 700 m water depth occurred above the Nile deep-sea fan between 10 and 6.5 ka BP while deeper cores recorded more stable anoxic conditions. These findings are further supported by other regional records and reveal time-transgressive changes in oxygenation state driven by rapid changes in primary productivity during a period of long-term deep-water stagnation.

Comment to Deoxygenation dynamics above the western Nile deep-sea fan during sapropel S1 at seasonal to millennial time-scales

“…Lines 473-475. The authors may cite a recent regional modelling study that simulated surface salinity anomaly distribution in the Mediterranean Sea by increasing Nile river discharge (Vadsaria et al, 2019).…”

Opening up the peer review process

“…3), with the criterion that the manuscript should be a research article. The manuscripts and articles, published with the Creative Commons Attribution 4.0 License, that the analysed text histories relate to are Blanchet et al (2021;acronym used in the examples TH1), Pawlak (2021; TH2) and Velasquez et al (2021;TH3). Analysing three manuscripts' text histories allows for comparison and contrast between the different manuscripts while still being a manageable amount of data to analyse.…”

Co-variations of climate and silicate weathering in the Nile Basin during the Late Pleistocene

“…These past humid periods were characterized by enhanced freshwater discharge and sediment export from the large African river systems to surrounding ocean margins (Blanchet et al, 2021;Mologni et al, 2020;Skonieczny et al, 2015). A number of recent studies conducted at a high temporal resolution (10-1000 years) in lake and deltaic sedimentary records across northern Africa suggested that gradual long-term monsoon oscillations had been often punctuated by millennial-scale episodes of hyperaridity (Bastian et al, 2017;Berke et al, 2012;Blanchet et al, 2020;Castañeda et al, 2016;Collins et al, 2013;Costa et al, 2014;Foerster et al, 2012;Liu et al, 2017;Tierney et al, , 2011bTierney et al, , 2011aTierney et al, , 2008Verschuren and Russell, 2009), as exemplified by significant increase in aeolian dust deposition in sediment records from African margins (Bouimetarhan et al, 2012;Collins et al, 2017Collins et al, , 2013Heinrich et al, 2021;McGee et al, 2013;Tierney et al, 2017). These hyperarid episodes occurred contemporaneously with North Hemisphere cooling events recorded in Greenland ice cores (i.e.…”