Deciphering Color Reflectance Data of a 520‐kyr Sediment Core From the Southern Ocean: Method Application and Paleoenvironmental Implications

Abstract

Sediment color reflectance contains important information on paleoenvironmental changes. Such data are routinely measured on sediment cores from the Southern Ocean. Their usefulness, however, is undervalued. Here for the first time, color reflectance of an ~520‐kyr sediment core retrieved off Prydz Bay (East Antarctica) was intensively investigated to correlate sediment color changes to changes in sediment compositions. Total carbon, total organic carbon, X‐ray fluorescence‐derived Fe/Ti and Br/Ti ratios, water content and mineral composition of sediment from this core, and color reflectance of another two sediment cores retrieved nearby were analyzed in this study. The raw reflectance data were transformed into their first‐order derivatives and subjected to factor analysis algorithms. A four‐factor model was selected to unmix the data set, explaining >80% of the total data variance. Its robustness is supported by similar factor analysis results from the other two cores. The four factors represent (from Factors 1–4) pulses of sedimentary Mn enrichment and Fe3+/Fe2+ changes in clay minerals, relative content changes of colorless components, hematite and goethite, and water, respectively. Scores of Factor 1 show high spikes at glacial terminations, recording enhanced deglacial ventilation in the abyssal Southern Ocean. Scores of Factor 2 mainly reflect biogenic components and chlorite but are further complicated by refractory terrigenous organic carbon and Mn‐oxides/‐hydroxides. Higher Scores of Factor 3 occur mainly at peak interglacial intervals, associated with enhanced transport of sediment of Antarctic origin by oceanic currents. Water content (Factor 4) in the core sediment was significantly influenced by opal content and sediment grain size compositions.

Evaluating Zr/Rb Ratio From XRF Scanning as an Indicator of Grain‐Size Variations of Glaciomarine Sediments in the Southern Ocean

“…Together, these observations support the robustness of the linear relationship between the ln(Zr/Rb) count ratio and the Zr/Rb concentration ratio. In addition, the consistent temporal variations of the ln(Zr/Rb) count ratio recorded in three Prydz Bay cores (i.e., P1‐2, P1‐3, and P4‐1; Figure 4) supports the general reliability of the ln(Zr/Rb) ratio as a proxy for regional paleoenvironmental change, because these cores were retrieved from the same sedimentary environment and are expected to record a similar pattern of past changes (Wu et al., 2017, 2019).…”

“…10.1029/2020GC009350 temporal variations of the ln(Zr/Rb) count ratio recorded in three Prydz Bay cores (i.e., P1-2, P1-3, and P4-1; Figure 4) supports the general reliability of the ln(Zr/Rb) ratio as a proxy for regional paleoenvironmental change, because these cores were retrieved from the same sedimentary environment and are expected to record a similar pattern of past changes (Wu et al, 2017(Wu et al, , 2019.…”

“…In addition, the elements Mn and Fe are both used in that multielement model, but can be significantly affected by their enrichment in oxide and oxyhydroxide phases. In the Southern Ocean, Mn‐oxides/hydroxides are typically authigenic in origin, and their formation may be affected by changes in abyssal ventilation (e.g., S. L. Jaccard et al., 2016; Jiménez‐Espejo et al., 2019; Presti et al., 2011; Wu et al., 2018), while Fe‐oxides/hydroxides can be associated with provenance changes (e.g., Wu et al., 2019). As such, these two elements may vary independent of sediment grain‐size in the Southern Ocean.…”

“…25–35 g freshly split samples were weighed, then dried at 55°C for 72 h and re‐weighed. Water content was calculated as follows (Wu et al., 2019): where WatCon represents water content (wt.%), and Mw and Md refer to the weights of wet and dried samples, respectively.…”

“…However, the association between sedimentary Zr/Rb ratio and grain‐size is not yet well established in marine sediments, especially in glaciomarine sediments, where sedimentation is influenced by multiple provenance sources and transport processes including wind, ocean currents, sea ice, and icebergs (Lamy et al., 2014, 2015; I. N. McCave et al., 2014; Weber et al., 2012, 2014). With the Southern Ocean increasingly viewed as a key area for understanding past and future global climate evolution (e.g., DeConto & Pollard, 2016; S. L. Jaccard et al., 2016; Schloesser et al., 2019; Sigman et al., 2010; Wilson et al., 2020), the development of new and convenient sedimentary proxies that can be applied in this region is an important target (e.g., Wu et al., 2019). In a few recent studies, the Zr/Rb ratio has been used as a proxy for past changes in sediment grain‐size, and hence bottom current speeds, in the vicinity of the Drake Passage (Lamy et al., 2015; Toyos et al., 2020) and offshore George V Land, East Antarctica (Campagne et al., 2015).…”