Oxygen, carbon, and clumped isotope compositions of detrital carbonates: A new combined proxy for quantifying relative sediment fluxes in carbonate terrains

At steady-state, sediment fluxes out of a drainage basin equal its average erosion rate. Quantifying relative sediment fluxes is therefore key in estimating spatial erosional variability among sub-basins and the consequential landscape evolution. Traditional approaches to quantify such fluxes in drainage basins include using the mineral and elemental compositions of sediments as markers for the relative contribution from sub-basins. Such an approach often fails to distinguish among bedrock sources, and have been shown to suffer from transport-related biases.

Here, we aim to test and explore the combination of these traditional approaches together with oxygen, carbon and ‘clumped’ isotope analyses of detrital carbonate as a novel combined proxy for relative sediment fluxes in carbonate-dominated drainage basins. We test this approach at the Hatrurim Syncline in southern Israel, east of the Dead Sea margins. The area comprises of marine carbonate rocks of the Judea Gr., as well as Hatrurim Fm. rocks that have experienced different grades of combustion-metamorphism, and thereby registered a wide range of isotope values together with distinctive carbonate mineral assemblages – allowing for using both ‘traditional’ and isotope-informed approaches. We collected bedrock and sediment samples from the Morag Basin in the Hatrurim Syncline, and analyzed their mineral and isotope compositions in bulk and specific grain-size fractions.

Our results show that: (a) Hatrurim Formation’s bedrock samples have a wide range of mineral and isotope values consistent with two main assemblages – high temperature metamorphic carbonates, and low temperature re-crystallized carbonates; and (b) Mineral and isotope compositions of fine grain sediment fractions (<2mm) show binary mixing between un-metamorphosed Judea Group and Low-T Hatrurim end-member sources. Coarser sediment fraction show deviations from a binary mixing, which we associate with contribution from a High-T Hatrurim third source.

Based on these analyses, we compiled a mixing model for fine grained sediments, aiming to identify the mineral and isotope compositions of end-member sources and to predict the mixing-ratio for each sediment sample. Model-predicted mixing ratios of sediment samples agree with mixing ratios estimated based on the relative exposure areas of the Judea Gr. and the low-T Hatrurim Fm. within the drainage area of each sediment sample. This consistency suggests that the Morag Basin is evolving under spatially uniform erosion conditions, in which sediment is being contributed equally from each area unit in the basin, and the overall landscape morphology is preserved over time.

A long-profile analysis of the Morag Basin channel network revealed several slope-break knickpoints, separating continuous channel sections with variable steepness indices. Accounting for our finding of a spatially uniform erosion rate, we interpret the knickpoints as reflecting transitions between different lithology-dependent rock erodibility rather than transient signals driven by tectonic or climatic perturbations. The Morag Basin thus presents a unique case where the morphology of the fluvial network has adjusted to erode the surface uniformly despite the multitude of rock types exposed in the basin.