Carbon and nitrogen isotope records of the Hirnantian glaciation

Abstract

The Hirnantian mass extinction was the second largest of the Phanerozoic. A global sea level fall resulting from a glaciation on Gondwanaland caused significant changes in ocean circulation patterns and nutrient cycling that is recorded as a worldwide positive δ¹³C excursion.

In chapter 2, carbon and nitrogen isotope profiles were reconstructed from two North American carbonate platforms in Nevada and one in the Yukon with the purpose of gaining a better understanding of proximal to proximal gradients in δ¹³C values from Hirnantian epeiric seaway sediment. Positive δ¹³C excursions are recorded in bulk inorganic and organic carbon fractions from all three sections, and in graptolite periderms from one section. A larger positive excursion is recorded in the proximal sediment (7‰) compared to proximal sediment (3-4‰). This gradient appears to reflect differences in surface water dissolved inorganic carbon δ¹³C values across epeiric seas. These findings are consistent with the carbonate weathering hypothesis, that predicts larger positive δ¹³C shifts in proximal settings of tropical epeiric seas resulting from changes in the local carbon weathering flux caused by the exposure of vast areas of carbonate sediment during glacioeustatic sea level fall and restricted shelf circulation. A 2‰ positive excursion in δ¹⁵N is interpreted to result from increased ocean ventilation, greater partitioning of atmospheric oxygen into downwelling surface waters, oxygen minimum zone shrinkage, and declining denitrification rates. This allowed for upwelling of recycled nitrogen with high δ¹⁵N values into the photic zone that forced exported organic matter from the photic zone to higher δ¹⁵N values, consistent with the observed positive shift in δ¹⁵N during the Hirnantian glaciation. This study presents a conceptual model to explain secular changes in δ¹³C and δ¹⁵N during the transition from a greenhouse to icehouse climate.

The second focus of this research, presented in chapter 3, was on improving the chemical and analytical methods for δ¹⁸O analysis of biogenic apatites. The technique applies cation exchange chromatography that allows for small sample sizes of apatite (200 µg) to be used for chemical conversion to Ag3PO4. The precision (0.15‰, 1σ) of δ¹⁸O analysis obtained using a Thermal Conversion Elemental Analyser Continuous Flow – Isotope Ratio Mass Spectrometer (TC/EA CF-IRMS), and the ability to collect multipe isotopes (O, Ca, Sr, REE) using a cation exchange column, makes this technique valuable for high-resolution, multi-isotope studies of biogenic apatites.