Microscale Alkenone Heterogeneity and Replicability of Ultra-High-Resolution SST Proxies in Laminated Marine Sediments

Alkenones in marine sediments are a key proxy for the reconstruction of past sea surface temperatures. Recent advances in mass spectrometry imaging (MSI) allow the Uk’37 proxy to be measured at the micrometer scale. Such high resolutions can theoretically provide a resolution similar to the observational record and hold the promise of reconstructing continuous climate records from subseasonal or interannual to centennial and millennial timescales. However, due to processes occurring during and after deposition, as well as the sampling and measurement procedures, it is unclear how much climate signal is preserved at these small spatial scales.

Here, we investigate this question using biomarker MSI on sediment records from the Santa Barbara Basin (SBB), a key site for Californian Current and Northeast Pacific SST reconstructions. We perform replicated MSI measurements on sediments with varying degrees of lamination to analyze the spatial structure and spatial reproducibility of the alkenone signal. 

We find that biomarker distributions are spatially heterogeneous even within laminae but exhibit small scale clustering. Spatial maps exhibit increased similarity in submm scale and with longer ranges and less overall variability within laminated horizons.

These findings have implications for the conversion of spatial biomarker data to time series reconstructions, as spatial heterogeneity patterns can act as an additional noise source during processing but can be accounted for using optimized 2D map to 1D timeseries conversion methods.

Using the replicated setup, we can derive the signal content and independent noise levels across varying sediment conditions. For our MSI-derived SST reconstructions this leads to signal-to-noise ratios ranging from ~1-3 integrated SNR at interannual resolution to ~4-6 integrated SNR at subdecadal (ENSO) timescales. The resulting SNRs can be used to infer optimal sampling strategies to tailor the resolution to the desired timescales of the studies for MSI based reconstructions as well as for discrete, traditional sampling efforts.

The findings imply that MSI based alkenone Uk’37 records at SBB during the Holocene and late Pleistocene, supported by careful noise & uncertainty estimations, can provide an upper limit for the reconstruction of SST variability beyond the instrumental record.