(Table S1) Globigerinoides sacculifer Boron isotope ratios of ODP Hole 165-999A

everal hypotheses have been put forward to explain the onset of intensive glaciations on Greenland, Scandinavia, and North America during the Pliocene epoch between 3.6 and 2.7 million years ago (Ma). A decrease in atmospheric CO2 may have played a role during the onset of glaciations, but other tectonic and oceanic events occurring at the same time may have played a part as well. Here we present detailed atmospheric CO2 estimates from boron isotopes in planktic foraminifer shells spanning 4.6-2.0 Ma. Maximal Pliocene atmospheric CO2 estimates gradually declined from values around 410 µatm to early Pleistocene values of 300 ?atm at 2.0 Ma. After the onset of large-scale ice sheets in the Northern Hemisphere, maximal pCO2 estimates were still at 2.5 Ma +90 µatm higher than values characteristic of the early Pleistocene interglacials. By contrast, Pliocene minimal atmospheric CO2 gradually decreased from 310 to 245 µatm at 3.2 Ma, coinciding with the start of transient glaciations on Greenland. Values characteristic of early Pleistocene glacial atmospheric CO2 of 200 ?atm were abruptly reached after 2.7 Ma during the late Pliocene transition. This trend is consistent with the suggestion that ocean stratification and iron fertilization increased after 2.7 Ma in the North Pacific and Southern Ocean and may have led to increased glacial CO2 storage in the oceanic abyss after 2.7 Ma onward.

Alkenone stable carbon isotopes, carbonate stable carbon and oxygen isotopes, planktic foraminifera boron isotopes and atmospheric CO2 calculations and box model results from Ras il-Pellegrin section, Malta

The development of a permanent, stable ice sheet in East Antarctica happened during the middle Miocene, about 14 million years (Myr) ago. The middle Miocene therefore represents one of the distinct phases of rapid change in the transition from the "greenhouse" of the early Eocene to the "icehouse" of the present day. Carbonate carbon isotope records of the period immediately following the main stage of ice sheet development reveal a major perturbation in the carbon system, represented by the positive d13C excursion known as carbon maximum 6 ("M6"), which has traditionally been interpreted as reflecting increased burial of organic matter and atmospheric pCO2 drawdown. More recently, it has been suggested that the d13C excursion records a negative feedback resulting from the reduction of silicate weathering and an increase in atmospheric pCO2. Here we present high-resolution multi-proxy (alkenone carbon and foraminiferal boron isotope) records of atmospheric carbon dioxide and sea surface temperature across CM6. Similar to previously published records spanning this interval, our records document a world of generally low (~300 ppm) atmospheric pCO2 at a time generally accepted to be much warmer than today. Crucially, they also reveal a pCO2 decrease with associated cooling, which demonstrates that the carbon burial hypothesis for CM6 is feasible and could have acted as a positive feedback on global cooling.

(Table 1) Results of pore water analyses, including boron content and boron isotope ratios

Drilling a transect of holes across the Costa Rica forearc during ODP Leg 170 demonstrated the margin wedge to be of continental, non accretionary origin, which is intersected by permeable thrust faults. Pore waters from four drillholes, two of which penetrated the décollement zone and reached the underthrust lower plate sedimentary sequence of the Cocos Plate, were examined for boron contents and boron isotopic signatures. The combined results show dilution of the uppermost sedimentary cover of the forearc, with boron contents lower than half of the present-day seawater values. Pore fluid "refreshening" suggests that gas hydrate water has been mixed with the sediment interstitial water, without profoundly affecting the d11B values. Fault-related flux of a deeply generated fluid is inferred from high B concentration in the interval beneath the décollement, being released from the underthrust sequence with incipient burial. First-order fluid budget calculations over a cross-section across the Costa Rica forearc indicate that no significant fluid transfer from the lower to the upper plate is inferred from boron fluid profiles, at least within the frontal 40 km studied. Expulsed lower plate pore water, which is estimated to be 0.26-0.44 km3 per km trench, is conducted efficiently along and just beneath the décollement zone, indicating effective shear-enhanced compaction. In the upper plate forearc wedge, dewatering occurs as diffuse transport as well as channelled flow. A volume of approximately 2 km3 per km trench is expulsed due to compaction and, to a lesser extent, lateral shortening. Pore water chemistry is influenced by gas hydrate instability, so that it remains unknown whether deep processes like mineral dehydration or hydrocarbon formation may play a considerable role towards the hinterland.

Boron isotope and B/Ca ratios of planktonic foraminifera

Here a new analytical methodology is described for measuring the isotopic composition of boron in foraminifera using multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). This new approach is fast (~10 samples analysed in duplicate per analytical session) and accurate (to better than 0.25 per mil at 95% confidence) with acceptable sample size requirements (1-3 mg of carbonate). A core top calibration of several common planktic and two benthic species from geographically widespread localities shows a very close agreement between the isotopic composition measured by MC-ICPMS and the isotopic composition of B(OH)-4 in seawater (as predicted using the recently measured isotopic equilibrium factor of 1.0272) at the depth of habitat. A down core and core top investigation of boron concentration (B/Ca ratio) shows that the partition coefficient is influenced by [CO2-3] complicating the application of this proxy. Nevertheless, it is demonstrated that these two proxies can be used to fully constrain the carbonate system of surface water in the Caribbean Sea (ODP Site 999A) over the last 130 kyr. This reconstruction shows that during much of the Holocene and the last interglacial period surface water at Site 999A was in equilibrium with the atmosphere with respect to CO2. During the intervening colder periods although the surface water pCO2 was lower than the Holocene, it was a minor to significant source of CO2 to the atmosphere possibly due to either an expansion of the eastern equatorial Atlantic upwelling zone, or a more local expansion of coastal upwelling in the southern Caribbean. Such reorganisation of the oceanic carbonate system in favour of a larger source of CO2 to the atmosphere from the equatorial ocean may require mechanisms responsible for lowering atmospheric CO2 during glacial periods to be more efficient than previously supposed.

Boron isotope composition and seawater characteristics of cold water scleratinian corals

The boron isotope systematics has been determined for azooxanthellate scleractinian corals from a wide range of both deep-sea and shallow-water environments. The aragonitic coral species, Caryophyllia smithii, Desmophyllum dianthus, Enallopsammia rostrata, Lophelia pertusa, and Madrepora oculata, are all found to have relatively high d11B compositions ranging from 23.2 per mil to 28.7 per mil. These values lie substantially above the pH-dependent inorganic seawater borate equilibrium curve, indicative of strong up-regulation of pH of the internal calcifying fluid (pH(cf)), being elevated by ~0.6-0.8 units (Delta pH) relative to ambient seawater. In contrast, the deep-sea calcitic coral Corallium sp. has a significantly lower d11B composition of 15.5 per mil, with a corresponding lower Delta pH value of ~0.3 units, reflecting the importance of mineralogical control on biological pH up-regulation. The solitary coral D. dianthus was sampled over a wide range of seawater pH(T) and shows an approximate linear correlation with Delta pH(Desmo) = 6.43 - 0.71 pH(T) (r**2 = 0.79). An improved correlation is however found with the closely related parameter of seawater aragonite saturation state, where Delta pH(Desmo) = 1.09 - 0.14 Omega(arag) (r**2 = 0.95), indicating the important control that carbonate saturation state has on calcification. The ability to up-regulate internal pH(cf), and consequently Omega(cf), of the calcifying fluid is therefore a process present in both azooxanthellate and zooxanthellate aragonitic corals, and is attributed to the action of Ca2+ -ATPase in modulating the proton gradient between seawater and the site of calcification. These findings also show that the boron isotopic compositions (d11Bcarb) of aragonitic corals are highly systematic and consistent with direct uptake of the borate species within the biologically controlled extracellular calcifying medium.