Mineralogical control on the fate of continentally derived organic matter in the ocean

First-order relationships between organic matter content and mineral surface area have been widely reported and are implicated in stabilization and long-term preservation of organic matter. However, the nature and stability of organomineral interactions and their connection with mineralogical composition have remained uncertain. In this study, we find that continentally derived organic matter of pedogenic origin is stripped from smectite mineral surfaces upon discharge, dispersal, and sedimentation in distal ocean settings. In contrast, organic matter sourced from ancient rocks that is tightly associated with mica and chlorite endures in the marine realm. These results imply that the persistence of continentally derived organic matter in ocean sediments is controlled to a first order by phyllosilicate mineralogy.

Temperature and moisture are minor drivers of regional-scale soil organic carbon dynamics

Storing large amounts of organic carbon, soils are a key but uncertain component of the global carbon cycle, and accordingly, of Earth System Models (ESMs). Soil organic carbon (SOC) dynamics are regulated by a complex interplay of drivers. Climate, generally represented by temperature and moisture, is regarded as one of the fundamental controls. Here, we use 54 forest sites in Switzerland, systematically selected to span near-independent gradients in temperature and moisture, to disentangle the effects of climate, soil properties, and landform on SOC dynamics. We estimated two SOC turnover times, based on bulk soil ¹⁴C measurements (τ_14C) and on a 6-month laboratory soil incubation (τ_i). In addition, upon incubation, we measured the ¹⁴C signature of the CO₂ evolved and quantified the cumulated production of dissolved organic carbon (DOC). Our results demonstrate that τ_i and τ_14C capture the dynamics of contrasting fractions of the SOC continuum. The ¹⁴C-based τ_14C primarily reflects the dynamics of an older, stabilised pool, whereas the incubation-based τ_i mainly captures fresh readily available SOC. Mean site temperature did not raise as a critical driver of SOC dynamics, and site moisture was only significant for τ_i. However, soil pH emerged as a key control of both turnover times. The production of DOC was independent of τ_i and not driven by climate, but primarily by the content of clay and, secondarily by the slope of the site. At the regional scale, soil physicochemical properties and landform appear to override the effect of climate on SOC dynamics.

Compound-Specific Radiocarbon Analysis by Elemental Analyzer-Accelerator Mass Spectrometry: Precision and Limitations

We examine instrumental and methodological capabilities for microscale (10-50 μg of C) radiocarbon analysis of individual compounds in the context of paleoclimate and paleoceanography applications, for which relatively high-precision measurements are required. An extensive suite of data for ¹⁴C-free and modern reference materials processed using different methods and acquired using an elemental-analyzer-accelerator-mass-spectrometry (EA-AMS) instrumental setup at ETH Zurich was compiled to assess the reproducibility of specific isolation procedures. In order to determine the precision, accuracy, and reproducibility of measurements on processed compounds, we explore the results of both reference materials and three classes of compounds (fatty acids, alkenones, and amino acids) extracted from sediment samples. We utilize a MATLAB code developed to systematically evaluate constant-contamination-model parameters, which in turn can be applied to measurements of unknown process samples. This approach is computationally reliable and can be used for any blank assessment of small-size radiocarbon samples. Our results show that a conservative lower estimate of the sample sizes required to produce relatively high-precision ¹⁴C data (i.e., with acceptable errors of <5% on final ¹⁴C ages) and high reproducibility in old samples (i.e., F¹⁴C ≈ 0.1) using current isolation methods are 50 and 30 μg of C for alkenones and fatty acids, respectively. Moreover, when the F¹⁴C is >0.5, a precision of 2% can be achieved for alkenone and fatty acid samples containing ≥15 and 10 μg of C, respectively.