Reply to Reviewer Comment RC3 on essd-2022-125

Abstract

The values and distribution patterns of the strontium (Sr) isotope ratio 87Sr / 86Sr in Earth surface materials is of use in the geological, environmental and social sciences. Ultimately, the 87Sr / 86Sr ratio of soil and everything that lives in and on it is inherited from the rock that is its parent material. In Australia, there are few large-scale surveys of 87Sr / 86Sr available, and here we report on a new, low-density dataset using 112 catchment outlet (floodplain) sediment samples covering 529,000 km2 of inland southeastern Australia (South Australia, New South Wales, Victoria). The coarse (< 2 mm) fraction of bottom sediment samples (depth ~0.6–0.8 m) from the National Geochemical Survey of Australia were milled and fully digested before Sr separation by chromatography and 87Sr / 86Sr determination by multicollector-inductively coupled plasma-mass spectrometry. The results show a wide range of 87Sr / 86Sr values from a minimum of 0.7089 to a maximum of 0.7511 (range 0.0422). The median 87Sr / 86Sr (± robust standard deviation) is 0.7199 (± 0.0112), and the mean (± standard deviation) is 0.7220 (± 0.0106). The spatial patterns of the Sr isoscape observed are described and attributed to various geological sources and processes. Of note are the elevated (radiogenic) values (≥~0.7270; top quartile) contributed by (1) the Palaeozoic sedimentary country rock and (mostly felsic) igneous intrusions of the Lachlan geological region to the east of the study area; (2) the Palaeoproterozoic metamorphic rocks of the central Broken Hill region; both these sources contribute radiogenic material mainly by fluvial processes; and (3) the Proterozoic to Palaeozoic rocks of the Kanmantoo, Adelaide, Gawler and Painter geological regions to the west of the area; these sources contribute radiogenic material mainly by aeolian processes. Regions of low 87Sr / 86Sr (≤~0.7130; bottom quartile) belong mainly to (1) a few central Murray Basin catchments; (2) some Darling Basin catchments in the northeast; and (3) a few Eromanga geological region-influenced catchments in the northwest of the study area; these sources contribute radiogenic material mainly by fluvial processes.