Posts Tagged 'paleo'

Push from the Pacific

Enhanced upwelling and CO2 degassing from the subpolar North Pacific during a warm event 14,000 years ago may have helped keep atmospheric CO2 levels high enough to propel the Earth out of the last ice age.

Continue reading ‘Push from the Pacific’

Deglacial upwelling, productivity and CO2 outgassing in the North Pacific Ocean

The interplay between ocean circulation and biological productivity affects atmospheric CO2 levels and marine oxygen concentrations. During the warming of the last deglaciation, the North Pacific experienced a peak in productivity and widespread hypoxia, with changes in circulation, iron supply and light limitation all proposed as potential drivers. Here we use the boron-isotope composition of planktic foraminifera from a sediment core in the western North Pacific to reconstruct pH and dissolved CO2 concentrations from 24,000 to 8,000 years ago. We find that the productivity peak during the Bølling–Allerød warm interval, 14,700 to 12,900 years ago, was associated with a decrease in near-surface pH and an increase in pCO2, and must therefore have been driven by increased supply of nutrient- and CO2-rich waters. In a climate model ensemble (PMIP3), the presence of large ice sheets over North America results in high rates of wind-driven upwelling within the subpolar North Pacific. We suggest that this process, combined with collapse of North Pacific Intermediate Water formation at the onset of the Bølling–Allerød, led to high rates of upwelling of water rich in nutrients and CO2, and supported the peak in productivity. The respiration of this organic matter, along with poor ventilation, probably caused the regional hypoxia. We suggest that CO2 outgassing from the North Pacific helped to maintain high atmospheric CO2 concentrations during the Bølling–Allerød and contributed to the deglacial CO2 rise.

Continue reading ‘Deglacial upwelling, productivity and CO2 outgassing in the North Pacific Ocean’

The influence of seawater carbonate chemistry, mineralogy, and diagenesis on calcium isotope variations in Lower-Middle Triassic carbonate rocks

The geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of carbonate rocks. As a result, calcium (Ca) isotope ratios (44Ca/40Ca, expressed as δ44/40Ca) can help to constrain ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. However, the δ44/40Ca of carbonate rocks is influenced not only by the δ44/40Ca of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonate δ44/40Ca in Upper Permian to Middle Triassic limestones (ca. 253 to 244 Ma) from south China and Turkey. This time interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawater δ44/40Ca may be expected based on several large carbon isotope (δ13C) excursions ranging from − 2 to + 8‰. Parallel negative δ13C and δ44/40Ca excursions were previously identified across the end-Permian extinction horizon. Here, we find a second negative excursion in δ44/40Ca of ~ 0.2‰ within Lower Triassic strata in both south China and Turkey; however, this excursion is not synchronous between regions and thus cannot be interpreted to reflect secular change in the δ44/40Ca of global seawater. Additionally, δ44/40Ca values from Turkey are consistently 0.3‰ lower than contemporaneous samples from south China, providing further support for local or regional influences. By measuring δ44/40Ca and Sr concentrations ([Sr]) in two stratigraphic sections located at opposite margins of the Paleo-Tethys Ocean, we can determine whether the data represent global conditions (e.g., secular variations in the δ44/40Ca of seawater) versus local controls (e.g., original mineralogy or diagenetic alteration). The [Sr] and δ44/40Ca data from this study are best described statistically by a log-linear correlation that also exists in many previously published datasets of various geological ages. Using a model of early marine diagenetic water-rock interaction, we illustrate that this general correlation can be explained by the chemical evolution of bulk carbonate sediment samples with different initial mineralogical compositions that subsequently underwent recrystallization. Although early diagenetic resetting and carbonate mineralogy strongly influence the carbonate δ44/40Ca values, the relationship between [Sr] and δ44/40Ca holds potential for reconstructing first-order secular changes in seawater δ44/40Ca composition.

Continue reading ‘The influence of seawater carbonate chemistry, mineralogy, and diagenesis on calcium isotope variations in Lower-Middle Triassic carbonate rocks’

Boron isotopes in foraminifera: systematics, biomineralisation, and CO2 reconstruction

The boron isotope composition of foraminifera provides a powerful tracer for CO2 change over geological time. This proxy is based on the equilibrium of boron and its isotopes in seawater, which is a function of pH. However while the chemical principles underlying this proxy are well understood, its reliability has previously been questioned, due to the difficulty of boron isotope (δ11B) analysis on foraminferal samples and questions regarding calibrations between δ11B and pH. This chapter reviews the current state of the δ11B-pH proxy in foraminfera, including the pioneering studies that established this proxy’s potential, and the recent work that has improved understanding of boron isotope systematics in foraminifera and applied this tracer to the geological record. The theoretical background of the δ11B-pH proxy is introduced, including an accurate formulation of the boron isotope mass balance equations. Sample preparation and analysis procedures are then reviewed, with discussion of sample cleaning, the potential influence of diagenesis, and the strengths and weaknesses of boron purification by column chromatography versus microsublimation, and analysis by NTIMS versus MC-ICPMS. The systematics of boron isotopes in foraminifera are discussed in detail, including results from benthic and planktic taxa, and models of boron incorporation, fractionation, and biomineralisation. Benthic taxa from the deep ocean have δ11B within error of borate ion at seawater pH. This is most easily explained by simple incorporation of borate ion at the pH of seawater. Planktic foraminifera have δ11B close to borate ion, but with minor offsets. These may be driven by physiological influences on the foraminiferal microenvironment; a novel explanation is also suggested for the reduced δ11B-pH sensitivities observed in culture, based on variable calcification rates. Biomineralisation influences on boron isotopes are then explored, addressing the apparently contradictory observations that foraminifera manipulate pH during chamber formation yet their δ11B appears to record the pH of ambient seawater. Potential solutions include the influences of magnesium-removal and carbon concentration, and the possibility that pH elevation is most pronounced during initial chamber formation under favourable environmental conditions. The steps required to reconstruct pH and pCO2 from δ11B are then reviewed, including the influence of seawater chemistry on boron equilibrium, the evolution of seawater δ11B, and the influence of second carbonate system parameters on δ11B-based reconstructions of pCO2. Applications of foraminiferal δ11B to the geological record are highlighted, including studies that trace CO2 storage and release during recent ice ages, and reconstructions of pCO2 over the Cenozoic. Relevant computer codes and data associated with this article are made available online.

Continue reading ‘Boron isotopes in foraminifera: systematics, biomineralisation, and CO2 reconstruction’

A pronounced fall in the CaCO3 saturation state and the total alkalinity of the surface ocean during the Mid Mesozoic

Geochemical models suggest that the calcium carbonate saturation state (ΩCaCO3) and total alkalinity (TA) of the ocean fluctuated considerably in the geologic past, under the influence of the continental weathering HCO3 source and the biological CaCO3 sink. The mid-Mesozoic is a key period in this respect because the advent of planktic calcifying organisms introduced the calcium carbonate compensation mechanism, buffering oceanic ΩCaCO3 and TA with respect to perturbations of the carbonate cycle. As a result, ΩCaCO3 and TA are thought to have fallen drastically in the mid-Mesozoic, and, excluding short-lived perturbations of the carbon cycle, to have remained similar to present values ever since. This important idea, put forward using geochemical models, is tested here using the geological record of marine calcified cyanobacteria. It is shown that under the most conservative of assumptions oceanic ΩCaCO3 must have been ≥6–10, and TA ≥3 mM, in the Permo-Triassic, prior to the advent of planktic calcifying organisms. It is likely, though, that oceanic ΩCaCO3 (≥10–16) and TA (≥4–6 mM) were greater than these minimum estimates during most of this time interval. In the Late Cretaceous, once the pelagic CaCO3 sink was well in place, oceanic ΩCaCO3 (~4) and TA (~2 mM) had fallen to values compatible with those of the modern ocean. This analysis implies a large fall in oceanic ΩCaCO3 and TA between 180 and 90 Myr, a time interval that encompasses the advent of planktic calcifying organisms and the introduction of the carbonate compensation mechanism. This cyanobacteria-based reconstruction strengthens the claim that pelagic calcification revolutionised the carbonate cycle. It also highlights, without implying a cause-effect link, that biocalcifying organisms evolved in an ocean with an elevated HCO3/H+ ratio, a condition known to favour biological calcification in laboratory experiments.

Continue reading ‘A pronounced fall in the CaCO3 saturation state and the total alkalinity of the surface ocean during the Mid Mesozoic’

Invariance of the carbonate chemistry of the South China Sea from the glacial period to the Holocene and its implications to the Pacific Ocean carbonate system

Highlights

  • The SCS carbonate chemistry didn’t change over the last glacial–interglacial cycle.
  • The SCS is not ocean dominated, or.
  • The Pacific carbonate chemistry did not change during the last GIT.
  • SCS CaCO3 content variations can be explained by altered lithogenic input.

Abstract

Substantial and correlated changes in marine carbonate (CaCO3) content of oceanic sediments commonly accompany the transitions from cold glacial periods to warm interglacial periods. The South China Sea (SCS) is said to be ocean-dominated at depth, and its CaCO3 records should reflect and preserve the effects of changes in the carbonate chemistry of the (western) Pacific Ocean. Using published and newly acquired CaCO3 data and a model for carbonate compensation dynamics, we show that a significant change with respect to carbonate saturation is unlikely to have occurred in the SCS during the last glacial–interglacial transition. Instead, the results from a carbonate deposition model argue that the saturation state of the SCS was largely invariant; a separate diagenetic model argues that changes in sediment CaCO3 content can be explained by alterations in lithogenic input. In turn, this could indicate that the carbonate ion concentration of the (western) Pacific at depths shallower than the sill to the SCS (ca. 2,400 m) has not changed appreciably between the last glacial period and the present interglacial.

Continue reading ‘Invariance of the carbonate chemistry of the South China Sea from the glacial period to the Holocene and its implications to the Pacific Ocean carbonate system’

Response to comment by Zeebe and Tyrrell on “the effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for the Eocene and Cretaceous ocean carbon chemistry and buffering”

Key points

  • Suspect parameter changed to new references
  • Main results of original study hold true
  • Demonstrated skill of MyAMI model, superior to existing correction factors

Continue reading ‘Response to comment by Zeebe and Tyrrell on “the effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for the Eocene and Cretaceous ocean carbon chemistry and buffering”’


Subscribe to the RSS feed

Powered by FeedBurner

Follow AnneMarin on Twitter

Blog Stats

  • 1,082,426 hits

OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

OUP book