Surface ocean warming and acidification driven by rapid carbon release precedes Paleocene-Eocene Thermal Maximum.

The Paleocene-Eocene Thermal Maximum (PETM) is recognized by a major negative carbon isotope (δ13C) excursion (CIE) signifying an injection of isotopically light carbon into exogenic reservoirs, the mass, source, and tempo of which continue to be debated. Evidence of a transient precursor carbon release(s) has been identified in a few localities, although it remains equivocal whether there is a global signal. Here, we present foraminiferal δ13C records from a marine continental margin section, which reveal a 1.0 to 1.5‰ negative pre-onset excursion (POE), and concomitant rise in sea surface temperature of at least 2°C and a decline in ocean pH. The recovery of both δ13C and pH before the CIE onset and apparent absence of a POE in deep-sea records suggests a rapid (< ocean mixing time scales) carbon release, followed by recovery driven by deep-sea mixing. Carbon released during the POE is therefore likely more similar to ongoing anthropogenic emissions in mass and rate than the main CIE.

The effect of matrix interferences on in situ boron isotope analysis by laser ablation multi-collector inductively coupled plasma mass spectrometry.

RATIONALE: Boron isotope analysis of marine carbonates by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) offers the potential for rapid sample throughput, and the means to examine micron-scale variations in the δ11 B signatures of fossil skeletons and shells/tests of marine organisms. Existing studies demonstrate an acceptable level of reproducibility is achievable, but also typically show a level of accuracy outside the limits required by most applications. Here we investigate matrix interference effects as a cause of inaccuracy and imprecision. METHODS: Analyses were performed on a standard format Thermo Scientific Neptune Plus MC-ICP mass spectrometer coupled to a New Wave Research 193 nm ArF laser ablation system. The effects of matrix interference on δ11 B analysis were investigated through analyses of a set of reference materials with differing B/Ca ratios. Three approaches to correct for matrix-induced effects were trialled: (1) use of matrix-matched standards, (2) utilisation of the relationship between δ11 B inaccuracy and11 B/43 Ca, 11 B/40 ArCa4+ or 11 B/Cainterference from three reference materials with known δ11 B values and varying B/Ca ratios, and (3) direct characterisation of the (sloping) interference itself. RESULTS: Matrix interference from scattered Ca ions on 10 B can impede both the accuracy and the reproducibility of δ11 B analysis by LA-MC-ICP-MS. Based on analyses of two in-house reference materials, deep sea coral PS69/3181 and inorganic calcite UWC-1, we find approach 2, following the 11 B/Cainterference relationship, gives the best mean accuracies (within 0.4‰ of solution values) and external reproducibilities (± 0.5‰ 2 SD for PS69/3181). This approach has been applied to analyses of an annual growth cycle of a Siderastrea siderea coral and eight Cibicidoides wuellerstorfi benthic foraminifera. Both coral and foraminifera data match solution MC-ICP-MS analyses within reported uncertainties. CONCLUSIONS: LA-MC-ICP-MS can produce accurate and precise δ11 B data to a 0.5‰ (2σ) level on <0.3 ng B after correction for Ca interference effects.

Capturing the global signature of surface ocean acidification during the Palaeocene-Eocene Thermal Maximum.

Geologically abrupt carbon perturbations such as the Palaeocene-Eocene Thermal Maximum (PETM, approx. 56 Ma) are the closest geological points of comparison to current anthropogenic carbon emissions. Associated with the rapid carbon release during this event are profound environmental changes in the oceans including warming, deoxygenation and acidification. To evaluate the global extent of surface ocean acidification during the PETM, we present a compilation of new and published surface ocean carbonate chemistry and pH reconstructions from various palaeoceanographic settings. We use boron to calcium ratios (B/Ca) and boron isotopes (δ11B) in surface- and thermocline-dwelling planktonic foraminifera to reconstruct ocean carbonate chemistry and pH. Our records exhibit a B/Ca reduction of 30-40% and a δ11B decline of 1.0-1.2‰ coeval with the carbon isotope excursion. The tight coupling between boron proxies and carbon isotope records is consistent with the interpretation that oceanic absorption of the carbon released at the onset of the PETM resulted in widespread surface ocean acidification. The remarkable similarity among records from different ocean regions suggests that the degree of ocean carbonate change was globally near uniform. We attribute the global extent of surface ocean acidification to elevated atmospheric carbon dioxide levels during the main phase of the PETM.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.