Abrupt sea surface pH change at the end of the Younger Dryas in the central sub-equatorial Pacific inferred from boron isotope abundance in corals (<i>Porites</i>)

Abstract

Abstract. The "δ 11 B-pH" technique was applied to modern and ancient corals Porites from the sub-equatorial Pacific areas (Tahiti and Marquesas) spanning a time interval from 0 to 20.720 calendar years to determine the amplitude of pH changes between the Last Glacial Period and the Holocene. Boron isotopes were measured by MultiCollector -Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) with an external reproducibility of 0.25‰, allowing a precision of about ±0.03 pH-units for pH values between 8 and 8.3. The boron concentration [B] and isotopic composition of modern samples indicate that the temperature strongly controls the partition coefficient K D for different aragonite species. Modern coral δ 11 B values and the reconstructed sea surface pH values for different Pacific areas match the measured pH expressed on the seawater scale and confirm the calculation parameters that were previously determined by laboratory calibration exercises. Most ancient sea surface pH reconstructions near Marquesas are higher than modern values. These values range between 8.19 and 8.27 for the Holocene and reached 8.30 at the end of the last glacial period (20.7 kyr BP). At the end of the Younger Dryas (11.50±0.1 kyr BP), the central sub-equatorial Pacific experienced a dramatic drop of up to 0.2 pH-units from the average pH of 8.2 before and after this short event. Using the marine carbonate algorithms, we recalculated the aqueous pCO 2 to be 440±25 ppmV at around 11.5 kyr BP for corals at Marquesas and ∼500 ppmV near Tahiti where it Correspondence to: E. Douville (eric.douville@lsce.ipsl.fr) was assumed that pCO 2 in the atmosphere was 250 ppmV. Throughout the Holocene, the difference in pCO 2 between the ocean and the atmosphere at Marquesas ( pCO 2 ) indicates that the surface waters behave as a moderate CO 2 sink or source (−53 to 20 ppmV) during El Niño-like conditions. By contrast, during the last glacial/interglacial transition, this area was a marked source of CO 2 (21 to 92 ppmV) for the atmosphere, highlighting predominant La Niña-like conditions. Such conditions were particularly pronounced at the end of the Younger Dryas with a large amount of CO 2 released with pCO 2 of +185±25 ppmV. This last finding provides further evidence of the marked changes in the surface water pH and temperature in the equatorial Pacific at the Younger Dryas-Holocene transition and the strong impact of oceanic dynamic on the atmospheric CO 2 content.

Intra-skeletal calcite in a live-collected Porites sp.: Impact on environmental proxies and potential formation process

“…Such ratios, equivalent to ~30 ppm of boron for the most calcite-rich samples (~30%) are clearly lower than values observed for modern Porites (Douville et al, 2010;Gaillardet and Allegre, 1995;Hemming and Hanson, 1992) but are similar to concentrations measured in some calcite benthic foraminifera (Rae et al, 2011;Yu et al, 2007;Yu et al, 2010) and in inorganic calcite (Gabitov et al, 2014).…”

“…(Douville et al, 2010;Hemming et al, 1998;Krief et al, 2010;Pelejero et al, 2005;Shinjo et al, 2013b;Wei et al, 2009) (Lécuyer et al, 2002;Penman et al, 2013).Benthic foraminifera calcite presentlower δ 11 B (14-17‰), except for the high-magnesium calcite of large foraminifera (26‰),whilehigher values (21-25‰) were measured on planktonic species (Holcomb et al, 2015;Hönisch et al, 2008;Rae et al, 2011;Sanyal et al, 1996). For gorgonian calcite corals, recent studies of the genus Keratoisis (bamboo coral) revealed low values of δ 11 B, between 14 and 17‰ (Farmer et al, 2015).…”

“…To test the reproducibility of the boron isotope analysis at various sizes, samples were systematically prepared twice,using 200 mg and 50 mg of material. The LSCE's batch protocol was used to purify and extract boron in final 0.1 N nitric acid solutions aspreviously described (Dissard et al, 2012;Douville et al, 2010). The 200 ppb-B solutions were introduced intothe mass spectrometer through a PFA-50 µL/minnebulizer and a micro-cyclonic chamber, limiting the blank contributionbelow 0.5%.…”

“…Contrarily, boron isotopes in Porites sp. skeleton has been used with success to reconstruct paleo-pH of surface seawater and ocean acidification related to the inception and development of the industrial era (e.g., Douville et al, 2010;Liu et al, 2009;Shinjo et al, 2013a;Wei et al, 2009;Wu et al, 2013). Even if boron isotopes in corals are now considered to be directly the reflect of pH of the calcification fluid (Allison et al, 2010;Blamart et al, 2007;Rollion-Bard et al, 2003;Venn et al, 2011), Trotter et al(2011) highlighted that pH of the calcification in scleractinian corals depends on pH of seawater.…”

Century-scale trends and seasonality in pH and temperature for shallow zones of the Bering Sea

“…Using these methods, long-term records of ocean pH variability have been established (15)(16)(17). However, these time series suffer from poor temporal resolution and do not resolve small-scale spatial heterogeneities of natural samples.…”