EARLY PALEOGENE STABLE ISOTOPE RECORDS AFTER ARTHUROSCOPIC DISSECTION

Thirteen years ago, over the course of a day at State College involving oxymoronic football and mid-caliber beer, Mike Arthur expressed to me a totally cool concept: “differential diagenesis”. According to Mike, when marine sediments comprised of carbonate and clay mixtures are buried to some depth, lithification involves carbonate shuttling from clay-rich carbonate-poor zones to clay-poor carbonate-rich zones on the sub-meter-scale via pressure solution; the most obvious manifestation being sharp boundaries between marl-limestone couplets in marine sedimentary sequences now uplifted and exposed on land.

Here I discuss stable isotope records across sections of Amuri Limestone in New Zealand. This formation consists of hundreds of dm-scale beds of limestone, siliceous limestone, marly limestone, and marl with mostly sharp contacts; it began as marine sediment deposited on a passive margin during the late Paleocene and early Eocene; the sediments were subsequently lithified, uplifted and folded, the latter during Neogene tectonism. Gorges now cut through Amuri Limestone roughly perpendicular to dip, providing sections with spectacular exposure. Circa 2016, sections of Amuri Limestone, such as at Mead and Branch streams, provide the most detailed d¹³C records across a 7 million year interval of widespread interest, from before the Paleocene-Eocene thermal maximum (PETM) through the initial Early Eocene Climatic Optimum (EECO). This includes a complete record of all possible carbon injection events, including those suggested as being “hyperthermals”, such as the PETM, H-1, I-1 and K/X events. The d¹⁸O records, however, have been reset during lithification.

That Amuri Limestone d¹³C records can be correlated with incredible fidelity to those generated using unlithified sediment recovered in deep-sea drill sites locations. Mass transfer must occur during lithification, but at the sub-meter-scale. The d¹³C signature remains, albeit muted, because of high rock-to-water ratio for carbon, and limited temperature-induced fractionation; the d¹⁸O signature is removed, because of low rock-to-water ratio for oxygen, and strong temperature-dependent fractionation. This has implications for utilizing rock sequences for understanding deep-time carbon cycling and climate.