Disrupted Coherence Between Upwelling Strength and Redox Conditions Reflects Source Water Change in Santa Barbara Basin During the 20th Century

Abstract

Varved sediments from the center of the Santa Barbara Basin (SBB) off southern California are a valuable archive for high‐resolution climate reconstruction. Using mass spectrometry imaging of lipid biomarkers, this study examines interactions of perennial upwelling strength and oxygen‐driven redox conditions at the sediment‐water interface from 1900 CE to 2009. In the SBB, variations in upwelling are recorded by isoprenoidal tetraethers from planktonic archaea, while the redox‐sensitive C29 stanol/stenol ratio reflects oxygen content in the bottom water and surficial sediment. The changing coherence of these two proxies allows investigation of the interplay between upwelling, bioproductivity and redox conditions, and their dependence on SBB source water composition during the 20th century. Prior to a large‐scale oceanographic regime shift observed in the North Pacific in the 1970s, both proxies are positively correlated: periods of enhanced upwelling promoted mixing, resulting in increased oxygen availability at the sediment‐water interface; conversely periods of reduced upwelling favored development of oxygen‐depleted water at depth. In the wake of the basin‐wide regime shift, changing oceanographic conditions led to a reduction in the southward flowing California Current and a stronger influence of the poleward California Undercurrent, which increased stratification and the supply of warm, oxygen‐poor tropical water in the subsurface. As a result, oxygen availability in SBB bottom waters was predominantly regulated by upwelling‐induced productivity and subsequent oxygen‐consuming remineralization of organic matter.

Hidden molecular clues in marine sediments revealed by untargeted mass spectrometry imaging

“…Four MSI datasets (Table 1) acquired from the uppermost ~10 cm of box core SPR0901-05BC collected from the center of SBB were employed to evaluate the data processing workflow proposed in this study. The sediment samples and MALDI-FT-ICR-MS analysis protocol have been described in detail in Alfken et al (2020Alfken et al ( , 2021. Datasets A1 and A2 were measured in Alfken et al (2020) with a mass window of 520-580 Da for mapping the U K′ 37 index, and datasets B1 and B2 were measured in Alfken et al (2021) with a wider mass window of 375-525 Da for mapping the stanol/stenol ratios.…”

“…The sediment samples and MALDI-FT-ICR-MS analysis protocol have been described in detail in Alfken et al (2020Alfken et al ( , 2021. Datasets A1 and A2 were measured in Alfken et al (2020) with a mass window of 520-580 Da for mapping the U K′ 37 index, and datasets B1 and B2 were measured in Alfken et al (2021) with a wider mass window of 375-525 Da for mapping the stanol/stenol ratios. A1 and B1 were obtained from the uppermost ~5 cm of the sediment, and A2 and B2 were obtained from 5-10 cm intervals on the sediment.…”

“…For the sake of brevity, A1 and A2, and B1 and B2 were merged, respectively, in this study, and the resulting new datasets were coded as datasets A and B. In addition, although datasets resulting from analysis of the m/z range 1280-1360 are available from Alfken et al (2021), they were deemed not to be practical for evaluating the untargeted workflow due to the low abundance of known biomarkers in this mass range.…”

“…Recently, we have increased the resolution of biomarker records in sediments to μm-scale by an extraction-free, mass spectrometry imaging (MSI)-based approach via matrix-assisted laser desorption/ionization coupled to Fourier transformationion cyclotron resonance-mass spectrometry (MALDI-FT-ICR-MS; Wörmer et al, 2014Alfken et al, 2019). In the annually varved sediment of the Santa Barbara Basin (SBB), such a fine resolution allows deciphering sub-annual distributions of archaeal tetraether lipids, haptophyte-derived alkenones, and sterols, which are sensitive to changes in upwelling intensity, sea surface temperature, and water column redox conditions (Alfken et al, 2020(Alfken et al, , 2021. The MSIbased protocol has also successfully revealed astonishingly diverse spatial signatures of lipid biomarkers along a ~1 cm thick microbial mat (Wörmer et al, 2020), demonstrating its capability in resolving fine spatial distributions of biomarker records beyond rather coarse concentration profiles.…”

“…In this study, we reported an untargeted data processing workflow for dissecting sedimentary MSI datasets and extracting information beyond conventional molecular proxies, enabling us to discover unique molecular signatures imprinted by varying biogeochemical processes in sediments. The proposed workflow is validated by re-analyzing the MSI datasets obtained from the SBB sediments measured by Alfken et al (2020Alfken et al ( , 2021. Untargeted peak detection, selection of informative peaks whose distributions mirror the lamination of the sediment, pattern recognition for molecular signature discovery, and relating molecular signatures to environmental variables are all part of the re-analysis.…”