Climate control on banded iron formations linked to orbital eccentricity

Abstract

Astronomical forcing associated with Earth's orbital and inclination parameters ("Milankovitch" forcing) exerts a major control on climate as recorded in the sedimentary rock record, but its influence in deep time is largely unknown. Banded iron formations, iron-rich marine sediments older than 1.8 billion years, offer unique insight into the early Earth's environment. Their origin and distinctive layering have been explained by various mechanisms, including hydrothermal plume activity, the redox evolution of the oceans, microbial and diagenetic processes, sea level fluctuations, and seasonal or tidal forcing. However, their potential link to past climate oscillations remains unexplored. Here we use cyclostratigraphic analysis combined with high-precision uranium-lead dating to investigate the potential influence of Milankovitch forcing on their deposition. Field exposures of the 2.48-billion-year-old Kuruman Banded Iron Formation reveal a well-defined hierarchical cycle pattern in weathering profile that is laterally continuous over at least 250 kilometres. The isotopic ages constrain the sedimentation rate at 10 m/Myr and link the observed cycles to known eccentricity oscillations with periods of 405 thousand and about 1.4 to 1.6 million years. We conclude that long-period, Milankovitch-forced climate cycles exerted a primary control on large-scale compositional variations in banded iron formations.

Figure 1. Rhythmic alternations in the weathering profile of the Kuruman BIF.

The alternations show a consistent pattern between different sections in the Griqualand West basin. Photos are from sections Whitebank (a), Prieska (b) and Woodstock (c).

Figure 2. Weathering profile logs and cyclostratigraphic correlations.

a, Logs of the Kuruman BIF weathering profile in sections Kuruman Kop (KK), Whitebank (WB), Woodstock (WS), Daniëlskuil (DK) and Prieska (PK). The relief ranges from 0 (= deeply weathered, unexposed) until 5 (= very much protuding). Blue zones mark our proposed correlations of the characteristic ridges (labelled a-d) and bundles (labelled 1 – 8) between the sections. Bandpass filter outputs are indicated on the righthand side of each log. ¹The Kleine Naute shale marks the bottom of the Kuruman Formation. However, its stratigraphic position with respect to our characteristic cycles is inferred from correlations with drill-core, because it does not crop out in the field. ²Bundles 7 - 8 are part of the uppermost Ouplaas Member of the Kuruman Formation in the Daniëlskuil – Kuruman area (Fig. S4). ³The Prieska log only captures the bottom 200 m of the Kuruman Formation in this region (Fig. S2, S4). b, Detail of cycles 2 – 4 in section WS showing lithological observations (left) and XRF results (right). Yellow colors indicate carbonate-dominated intervals, blue colors the oxide-dominated (see Supplementary Fig. S3 for a complete legend).

Figure 3. Spectral analysis results.

a, MTM power spectra (three 2π prolate tapers) for the logs from sections KK, WB and WS (10 cm linear interpolated; detrended) with AR1 background, 90, 95 and 99% confidence levels and peak labels in metres. Blue zones indicate the two areas of enhanced spectral power that are linked to the characteristic alternations and bundles. The peaks at 1 - 3 m are considered artefactual harmonics (Supplementary Fig. S7). b, EHA amplitude (3 2π; 30 m window) and MTM power spectrum (top) for the log of section WB (left). White arrows mark the frequency shift of the two main high amplitude components between cycle 3b and 5a (mean periods are indicated in metres).

Figure 4. U-Pb zircon ages and depositional rate.

a, Concordia diagram showing the concordant high-precision U-Pb TIMS results and the calculated ages. The grey band represents the uncertainty on the Concordia curve due to the decay constant uncertainties. The ellipses represent the U-Pb isotopic data and 2σ uncertainty for individual chemically abraded zircon fragments, and the colours represent the different samples. The ²⁰⁷Pb/²⁰⁶Pb ages are given for each sample, either as a weighted mean or from an individual analysis, along with the depth in metres in the UUBH-1 core. Left: schematic representation of sample depth (stilpnomelane lutite L1 – L4) and extent of the Kuruman Formation (in grey) in the UUBH-1 drill-core (see Methods). b, Depositional rate model for the Kuruman BIF with 97.5% confidence intervals indicated.