Carbon Isotope Stratigraphy of the ~2.5 Gyr old Dale's Gorge Member, Western Australia: Correlation Potential and Paleoclimatic Significance

Abstract

Banded Iron Formations (BIFs) are chemical sedimentary formations made up of iron-rich and iron-poor beds on different scales (Trendall 1968) belonging to the Precambrian era. The Dale’s Gorge Member (DGM) of the Brockman Iron Formations in Western Australia is one of the oldest BIF’s, leading back to ~2.5 Ga. It presented an excellent means to understand the changes that took place during the early evolution of the Earth. Recently a climate control, as a primary forcing mechanism modulated by long (~405 kyr) and very long period (1.6 Myr) eccentricity cycles were proposed for the formation of the large- scale bands within the Kuruman Iron Formation (KIF) of the Transvaal Basin, South Africa (Lantink et al. 2019). It is temporally equivalent to Dale’s Gorge Member (Trendall et al. 2004; Pickard 2003). Stable isotope analysis along with elemental data was used along with cyclostratigraphic analysis to test if the geochemistry of the formation also suggested this relationship for the Dale’s Gorge Member. Samples were taken at a ~1m interval for the carbon isotope analysis, to establish the larger-scale pattern. δ13Ccarb values between -2.2 ‰ to -12 ‰ (VPDB) were observed in the DGM. Spectral Analysis and Bandpass filtering provided evidence for cyclicity with the isotope record as well as elemental data. The spatial wavelength of ~4.7 m and 17.5 m were noted that could be related to ~405 kyr and ~1.1-1.6 Myr eccentricity cycles with slightly reduced periodicities. Another spatial wavelength of ~8.7 m appeared to be made of a half 17.5 m cycle and/or double 4.7 m cycle. It also suggested a relationship to sedimentation changes, and possibly a relation to eccentricity forcing of the ~688 kyr cycle. Analysis from other proxies taken from the same area further corroborated this evidence. This may suggest sensitivity to local conditions. Similarities in the carbon isotope analysis were used to test the probability of carbon isotope stratigraphy between Dale’s Gorge Member and the Kuruman Iron Formation. It was noted that despite their similarities, the isotope records were hard to correlate and the primary stratigraphy was better established using mineralogical proxies. The carbon isotope records were then correlated using association illustrated from the banding relationships already established. The relationships noted with the isotope and mineralogy were used to fine-tune the correlation on the ~4.7m macrobanding level. At least 8 large scale cycles correlated to the very long period eccentricity forcing were recorded within both the Dale’s Gorge Member and the Kuruman Iron Formation. The relationships observed between the elemental variations, banding and carbon isotope values along with the eccentricity relationship indicate the variations observed in the BIF’s could be traced to changes in the water column or early diagenetic DIR formation process. The more depleted δ13Ccarb values could be related to iron-rich bands with low Corg, which suggested a preferential early diagenetic model using dissimilatory iron reduction. Correlation to eccentricity forcing, signal a low latitude climate control related to monsoon intensity, and bio- productivity. The differences observed in the isotope records of the DGM and the KIF and their similar formation processes imply that the distance to the continent and depth of the basin affect the type of rock formation.