Evaluating the Reliability of Grain‐Size Sorting for Organic Biomarker Analysis

Abstract Organic and inorganic geochemical proxies are fundamental for interpreting past climate variability. For inorganic analysis, marine sediments are often size fractionated to isolate foraminifera, while organic analysis employs subsamples of bulk sediment. Although previously sieved sediments could be used for biomarker analysis, this approach and potential methodological biases have not been assessed in pre‐Holocene sediments. Here we explore lipid biomarkers (isoGDGTs, brGDGTs, n ‐alkanes) across grain‐size fractions in Cenozoic marine sediments, assessing whether sieved sediments can be used to reconstruct paleoenvironmental change. We analyze biomarkers in fine (<63 μm) and coarse (>63 μm) grain‐size fractions and compare various proxy metrics with those in bulk sediments. Our findings reveal a strong linear correlation between TEX 86 in different size‐fractions and bulk sediment ( r 2  = 0.85–0.94, both p  < 0.001), indicating sea surface temperature (SST) reconstructions are viable using previously sieved sediments. MBT′ 5ME shows a weaker yet significant correlation between fine and bulk sediment ( r 2   = 0.70–0.81, both p  < 0.001). In fine fractions, n ‐alkanes typically reflect the bulk sediment composition, whereas coarse sediments consistently underestimate bulk sediment chain length metrics, suggesting organo‐mineral associations. Our results demonstrate GDGT‐based temperature proxies can typically be analyzed using sieved sediments, whilst n ‐alkanes can be influenced by organo‐mineral associations and contamination during processing. We assessed contamination from traditional processing techniques, determining sieving sediments into plastic beakers can introduce hydrocarbon contamination but this does not affect GDGT analysis. Our study confirms that archives of fine‐grained sediment fractions can be resurrected for GDGT analysis, encouraging a re‐examination of global stores of processed samples.