1748-9326/14/2/025007

Arguably among the most globally impactful climate changes in Earth's past million years are the glacial terminations that punctuated the Pleistocene epoch. With the acquisition and analysis of marine and continental records, including ice cores, it is now clear that the Earth's climate was responding profoundly to changes in greenhouse gases that accompanied those glacial terminations. But the ultimate forcing responsible for the greenhouse gas variability remains elusive. The oceans must play a central role in any hypothesis that attempt to explain the systematic variations in pCO 2 because the Ocean is a giant carbon capacitor, regulating carbon entering and leaving the atmosphere. For a long time, geological processes that regulate fluxes of carbon to and from the oceans were thought to operate too slowly to account for any of the systematic variations in atmospheric pCO 2 that accompanied glacial cycles during the Pleistocene. Here we investigate the role that Earth's hydrothermal systems had in affecting the flux of carbon to the ocean and ultimately, the atmosphere during the last glacial termination. We document late glacial and deglacial intervals of anomalously old 14 C reservoir ages, large benthic-planktic foraminifera 14 C age differences, and increased deposition of hydrothermal metals in marine sediments from the eastern equatorial Pacific (EEP) that indicate a significant release of hydrothermal fluids entered the ocean at the last glacial termination. The large 14 C anomaly was accompanied by a ~4-fold increase in Zn/Ca in both benthic and planktic foraminifera that reflects an increase in dissolved [Zn] throughout the water column. Foraminiferal B/Ca and Li/Ca results from these sites document deglacial declines in [ ] throughout the water column; these were accompanied by carbonate dissolution at water depths that today lie well above the calcite lysocline. Taken together, these results are strong evidence for an increased flux of hydrothermally-derived carbon through the EEP upwelling system at the last glacial termination that would have exchanged with the atmosphere and affected both Δ 14 C and pCO 2 . These data do not quantify the amount of carbon released to the atmosphere through the EEP upwelling system but indicate that geologic forcing must be incorporated into models that attempt to simulate the cyclic nature of glacial/interglacial climate variability. Importantly, these results underscore the need to put better constraints on the flux of carbon from geologic reservoirs that affect the global carbon budget.

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1. Introduction-recent discoveries and motivation for this study

There have been several hypotheses put forth over the years to account for the systematic variations in atmospheric pCO 2 that accompanied glacial/interglacial cycles during the Pleistocene (Berger 1982, Archer et al 2000, Sigman and Boyle 2000, Lund and Asimow 2011, Stott and Timmermann 2011, Broecker et al 2015, Huybers and Langmuir 2017). Yet, after more than four decades of scientific effort to evaluate these hypotheses and identify the ultimate cause or causes of the greenhouse gas variations, the answer remains elusive. This likely reflects the fact that no single mechanism acting alone appears capable of explaining all aspects of carbon cycle behavior that accompanied the glacial cycles (Fischer et al 2010). For example, recent discoveries indicate marine carbon budgets underestimate the amount of carbon entering the oceans from hydrothermal systems. This is because over the past two decades reservoirs of liquid and hydrate CO 2 have been discovered in rocks and sediments that blanket the margins of some active marine hydrothermal vents (Sakai et al 1990, Inagaki et al 2006, Lupton et al 2006, Lupton et al 2008). In the back arc basin of the Okinawa Trough a 'lake' of liquid CO 2 accumulates beneath a cap of CO 2 -hydrate (Inagaki et al 2006, Nealson 2006) and at shallower ocean depths pure CO 2 gas is observed to emanate directly from sediments that blanket the margins of hydrothermal vents (figure 1). Pools of liquid CO 2 have been discovered in the Aegean Sea (Camilli et al 2015). Geologic carbon reservoirs such as these are not explicitly included in current marine carbon budgets. In fact, quantifying how carbon from sediment and rock-hosted reservoirs contributes to the overall marine carbon budget is not yet possible because there are too few observations and virtually no in situ measurements of the CO 2 flux from these types of reservoirs (Burton et al 2013). Yet, in one remarkable study, Lupton et al (2008) were able to quantify the flux of liquid CO 2 from a sediment/rock-hosted reservoir at a site on Mariana trench in the western Pacific by observing the rate that bubbles of pure liquid CO 2 emanate from a small opening on the seafloor. They estimated the carbon flux from this single opening to be 0.1% of the entire Mid Ocean Ridge flux. When Lupton et al (2008) documented the gas and liquid phases in the western Pacific, they pointed out that of the hundreds of vents thus far investigated, only a few were found to have a separate CO 2 phase, concluding this must be a rare circumstance and perhaps limited to volcanic arcs. However, investigations targeting CO 2 fluxes from the cool flanks of active hydrothermal sites has not been a priority and observations are therefore sparse (Burton et al 2013). With only about one-third of the global ocean spreading ridges surveyed, vast portions of the sea floor have not been investigated at all (Beaulieu et al 2015). Even if only a small percentage of the unsampled hydrothermal systems contain separate gas or liquid CO 2 phases it could change the global marine carbon budget substantially (Burton et al 2013). Importantly, where these reservoirs have been found at shallow-intermediate depths, the flux from the sediment and rock reservoirs is modulated by a density-stratified boundary layer at the sediment/water interface where a thin layer of CO 2 saturated-seawater and CO 2 hydrate forms (Inagaki et al 2006, Lupton et al 2006). The boundary layer is sensitive to temperature changes. Reservoirs of carbon like this can exist at deep water depths but may be less sensitive to temperature changes. Figure 1. Nearly pure CO 2 bubbles emanating from sediments that blanket an active hydrothermal system in the western tropical Pacific. Photos by Roy Price, courtesy of Jan Amend. Download figure: Standard image High-resolution image Export PowerPoint slide Stott and Timmermann (2011) were motivated by the observations described above to consider whether the large deglacial radiocarbon Δ14C excursions documented from biogenic carbonates in the eastern Pacific and Indian Oceans (Marchitto et al 2007, Stott et al 2009, Bryan et al 2010) could be indicative of past episodes of geologic carbon release from hydrothermal sources. The magnitude of the deglacial Δ14C excursions in the eastern equatorial Pacific (EEP) core VM21-30 are too large to be explained by ventilation of a previously isolated deep-water mass. Furthermore, similarly large Δ14C anomalies are not found at other intermediate depth sites bathed by the same water mass (De Pol-Holz et al 2010, Ronge et al 2016). Given this, Stott and Timmermann (2011) reasoned that the magnitude and duration of the excursion in the EEP would have required a sustained input of 14C-dead carbon from a localized source, the most likely being a hydrothermal source in the EEP. There are several volcanic provinces located within the EEP that could have potentially contributed carbon-rich hydrothermal fluids, including the Galapagos Hot Spot (Christopher et al 2013, Harpp and White 2018), the Galapagos Spreading Center, the Cocos Ridge, the Carnegie Ridge and perhaps the submerged margins of the Central American Arc. If a substantial release of geologic carbon to the upper ocean occurred in the EEP from one or more of these volcanic sources, it has important implications for reconciling why there was a large decrease in atmospheric Δ14C during the glacial termination when other cosmogenic radionuclides did not (Muscheler et al 2004, Petrenko et al 2016) because the EEP is a primary conduit for exchange of carbon from the ocean to the atmosphere (Takahashi et al 2009). The original Stott and Timmermann (2011) hypothesis met with considerable skepticism because few paleoceanographers were aware of the recent discoveries of geologic reservoirs of CO 2 in the ocean or that hydrothermalism may have varied on glacial/interglacial timescales. But over the past several years other studies have begun to consider ways that geologic carbon could have influenced glacial/interglacial CO 2 and climate variability (Lund and Asimow 2011, Broecker et al 2015, Tolstoy 2015, Lund et al 2016, Ronge et al 2016, Huybers and Langmuir 2017). The seafloor discoveries and the hypotheses summarized above pose challenges to the paleoceanography community to effectively evaluate whether geologic processes contributed significantly to the systematic variations in atmospheric pCO 2 that accompanied each glacial/interglacial cycle over the past million years. In the present study we evaluated further the suggestion by Stott and Timmermann that the large Δ14C anomalies in the EEP were due to a release of hydrothermal carbon during the last glacial termination. We emphasize that the current study does not attempt to identify the specific site of hydrothermal carbon release. There are multiple sites in the EEP where hydrothermal and volcanic activity is known. And while there have been various studies of active hydrothermalism in the EEP, there is a significant amount of area in the EEP that has not been explored. The Galapagos island chain itself is an active volcanic center as is the EPR. The flanks of the Central American Arc are also known to emit large amounts of CO 2 through the sediments that blanket the margins of the arc (Salazar et al 2001). Hence, there are large gaps in knowledge about where CO 2 is emitted in the EEP. We also emphasize that this study is not an attempt to identify a specific phase of CO 2 released to the ocean at the glacial termination. That topic is beyond the scope of this effort. The primary intent here is to further evaluate the suggestion by Stott and Timmermann (2011) that hydrothermal carbon was released to the ocean in the EEP during at the last glacial termination and hydrothermal carbon contributed to the very large Δ14C anomalies observed in marine sediments. The Stott and Timmermann hypothesis made several predictions about carbonate chemistry and carbonate preservation in the EEP during the last glacial termination. An evaluation of those predictions here is one step in an effort to evaluate if and how geologic sources of carbon contributed significantly to the glacial/interglacial carbon cycle variability.

2. Methods

A complete description of the analytical methods used in this study is provided in the supplementary section. The Stott and Timmermann hypothesis predicts that if there was a release of hydrothermal carbon of sufficient quantity to produce the large 14C age anomalies in the EEP it would have changed the carbonate chemistry and affected calcite and aragonite preservation. It may have also left a trace metal fingerprint of hydrothermal elements on the sediments. We demonstrate that large benthic-planktic 14C age differences originally documented in the VM21-30 core by Stott et al (2009) are observed in other cores from the EEP (figure 2; table 1). We also demonstrate that the surface ocean reservoir age varied significantly across the EEP during the glacial termination with the largest increase in surface ocean reservoir age of more than 6000 years, just south of the equator within the zone of upwelling. Figure 2. Location of cores used in this study. Lambert projection of bathymetry is from m_Map. Download figure: Standard image High-resolution image Export PowerPoint slide Table 1. Sediment cores used in the study. Core location Latitude Longitude Water depth (m) VM21-30 −1.22 −89.68 617 VM21-29 0.95 −89.35 712 ODP 1242A 7.86 −106.61 1364 Variability in the ocean carbonate system and carbonate preservation has been evaluated using scanning electron microscopy to inspect carbonates for signs of dissolution. The weight and abundance of Orbulina universa, a planktic foraminifera, is quantified because changes in the shell weight of individual O. universa has been shown to correlate positively with carbonate ion concentration [ ] (Bijma et al 1999, Bijma et al 2002). B/Ca, Li/Ca, and Zn/Ca of planktic and benthic foraminifera were measured to assess changes in ambient seawater carbonate chemistry. B/Ca has been investigated as a proxy for carbonate system parameters in planktic foraminifera (Yu et al 2007, Foster 2008, Hendry et al 2009, Allen et al 2012, Yu et al 2013, Holland et al 2017, Howes et al 2017, Quintana Krupinski et al 2017). B/Ca is particularly useful because in some species it is unaffected by dissolution or temperature (Wara et al 2003, Foster 2008, Henehan et al 2015, Dai et al 2016, Haynes et al 2017, Quintana Krupinski et al 2017), including Neogloboquadrina dutertrei and Neogloboquadrina incompta that are studied here. Li/Ca has been less utilized as a proxy for [ ] because of the opposing effects of [ ] and temperature on Li incorporation into the foraminiferal calcite. Similar to B/Ca, Li/Ca has been shown to have a positive correlation with carbonate saturation state ( defined as = − ) although unlike B/Ca (which is not influenced by temperature), Li/Ca exhibits an inverse relationship with temperature (Hall and Chan 2004, Marriott et al 2004, Lear and Rosenthal 2006, Bryan and Marchitto 2008, Lear et al 2010, Doss et al 2018). Therefore, concomitant changes in temperature and Δ[ ] can be obfuscated in Li/Ca, even though the Li/Ca-temperature relationship is weaker in planktic foraminifera (Hall and Chan 2004, Hathorne and James 2006). When paired with temperature proxies (Mg/Ca, alkenones) and other carbonate system proxies (e.g. B/Ca), Li/Ca can be used to deconvolve various influences. Core top studies have suggested that Zn/Ca in benthic foraminiferal calcite also covaries with [ ] (Marchitto et al 2000, 2002, van Dijk et al 2017). However, unlike Li and B, Zn behaves non-conservatively in the ocean and has a refractory nutrient-type distribution with a relatively shorter oceanic residence time (<50 kyr). Additionally, Zn incorporation is proportional to seawater [Zn] (Boyle 1981, Marchitto et al 2000, Marchitto et al 2002), and therefore, Zn/Ca is sensitive to both [ ] and changes in seawater [Zn]. This is important in the present study because [Zn] sw is a tracer of hydrothermal flux (Levin et al 2016). However, in low calcite-saturation conditions (e.g. some benthic foraminiferal environments), under-saturation (Δ[ ]) can also affect Zn/Ca. Therefore, to use Zn/Ca to reconstruct past ocean [Zn] sw requires an independent estimate of seawater [ ] (Marchitto et al 2002, van Dijk et al 2017). For this reason, we combine measurements of Zn/Ca with both B/Ca and Li/Ca to deconvolve the influences of seawater [Zn] from changes in [ ]. We quantified changes in accumulation of hydrothermal elements in oxide overgrowths precipitated on the carbonate sediments in the EEP by analyzing uncleaned foraminifera from the late glacial and deglacial sections of two cores. This is not an effort to quantify actual fluxes of hydrothermal metals but rather to assess whether there was increased accumulation of hydrothermal metals across the glacial termination.

3. Results

4. Discussion

In the late 1970s the plate tectonics theory gained strong support from oceanographic data collected by the Deep-Sea Drilling Program and other initiatives that acquired sea floor observations, including magnetic reversals and biostratigraphic data that documented increasing ages of the sediments overlying basaltic crust away from the mid-Atlantic mountain chain. The discovery of 'thermal springs' along the Galapagos Rift during the late 1970s (Corliss et al 1979) still stands as one of the most transformative discoveries of that era. That initial discovery at the Galapagos Rift did not quantify the actual flux of CO 2 emanating from the 'thermal springs'. But the measurements of alkalinity and pH indicated those springs carried a significant amount of CO 2 (Corliss et al 1979). It has taken many decades since that original discovery to obtain what is still a very sparse database of observations from the boundaries of active tectonic plates where much of the geologic activity and hydrothermalism occurs. That sparse observational database leaves considerable uncertainty about how vents contribute to the overall carbon budget of the oceans. It is in this long-term context of exploration that the discovery of liquid and solid CO 2 reservoirs stands-out as an important advance. These discoveries, together with geologic observations of variable hydrothermalism during the Pleistocene glacial cycles make it clear that processes that regulate the flux of carbon from the Earth's interior to its exterior are not spatially uniform nor are they necessarily slow and continuous. Thus, there remains much to learn about what regulates the flux and accumulation of hydrothermal carbon. The question of whether the accumulation and variable flux of geologic carbon to the ocean from these sources influenced the global carbon cycle on glacial/interglacial time scales remains an incipient hypothesis to be tested. Towards that goal, we evaluated whether there was increased flux of hydrothermal carbon to the ocean during the last glacial termination. The findings presented here are compelling evidence that there was increased flux of hydrothermally-derived carbon to the ocean in the EEP at the last glacial termination. This has important implications for the history of atmospheric pCO 2 and Δ14C because the EEP is a primary conduit for exchange of carbon between the ocean to the atmosphere. The results do not put constraint on the amount of hydrothermal carbon released to the surface ocean in the EEP during the last glacial termination but the magnitudes of change in radiocarbon, δ13C and trace elements implies a large amount of carbon was released over thousands of years that would have contributed to the higher ΔpCO 2 documented in boron isotope records (Martinez-Boti et al 2015). The dissolution of carbonates at 600 meters in the EEP alone is strong evidence that there was large flux of carbon into the upper ocean that altered the carbonate chemistry. The O. universa shell weight data also imply a drop of surface ocean [ ] that may have been as much as 50%. The planktic foraminiferal B/Ca and Li/Ca data presented here also indicate a large increased flux of carbon into the surface ocean that lowered the carbonate saturation. Together with boron isotope results, our findings indicate geologic carbon would have been a significant fraction of the carbon released from the ocean to the atmosphere at the last glacial termination. In the modern ocean the VM21-29 and VM21-30 sites are above the calcite lysocline and close to the aragonite lysocline in the EEP. The variable dissolution evident in benthic foraminifera implies that the carbonate saturation in the EEP intermediate waters dropped sometimes during the late glacial and deglaciation. This implies episodic upwelling of 14C-depleted, CO 2 -rich waters in the EEP during the late glacial and early deglacial. This is consistent with large range of 14C ages and very old reservoir ages among planktic foraminifera within the late glacial and deglacial samples. The large B-P 14C age anomalies and increased surface ocean reservoir ages together with the increased accumulation of hydrothermal metals is evidence that the source of excess carbon that affected the carbonate changes was from a hydrothermal source and not from respired metabolic carbon. The large enrichment of Zn in both the foraminiferal calcite and in oxide coatings, along with the enrichment and co-variation of Fe and V indicates there was a hydrothermal source nearby. The oxide coatings would have formed after deposition at the sea floor. However, the elevated Zn/Ca in the cleaned planktic and benthic foraminifera of VM21-30 in the deglacial section indicates there was a large enrichment of dissolved Zn throughout the water column. The most likely source of this Zn would be from hydrothermal systems. However, it is not possible to infer from what hydrothermal system the Zn came. The fact that Zn is elevated throughout the water column over the deglacial implies there was a persistent supply. And this input of Zn coincided with the input of 'old' carbon. At the present time there is no known occurrence of liquid CO 2 at the hydrothermal vents in the EEP or on the margins of the Galapagos that might act as a supply of these metals during the deglacial. But it is worth noting that supercritical CO 2 (SFC-CO 2 ) is a highly efficient solvent for extraction of heavy metals (Lin et al 2014). Therefore, if there was SCF-CO 2 formed at depth beneath the hydrothermal or volcanic systems in the EEP, it could act as an efficient source for Zn and other metals. This is obviously a question that will require additional research. But the elevated Zn/Ca and 'very old' carbon recorded by carbonates in the EEP points to a geologic source of carbon. The results presented here point to a large amount of hydrothermal carbon released at times during the last glacial termination and that carbon upwelled to the surface, but the changes in carbonate chemistry were likely also responding to changes in the upwelling intensity (Koutavas et al 2002, Koutavas and Lynch-Stieglitz 2003, Koutavas and Sachs 2008) that indicates stronger upwelling. The stronger upwelling documented in previous studies and the geochemical anomalies documented here coincided with elevated ΔpCO 2 in the EEP (Martinez-Boti et al 2015). It therefore appears that there were two factors affecting the flux of carbon to the surface ocean and atmosphere from the EEP during the glacial termination: episodic release of geologic carbon to the intermediate waters, and intensified upwelling. Because the EEP is one of the primary conduits for exchange of carbon from the ocean to the atmosphere our findings require reconsideration of the prevailing view about where the ocean released excess carbon to the atmosphere at the glacial termination. The results also underscore the need to investigate other sites for evidence of geologic carbon release. The present study does not put a constraint on the mechanism responsible for the increased flux of hydrothermal carbon at the glacial termination. The two prevailing ideas, sea level forcing (Lund and Asimow 2011, Huybers and Langmuir 2017) and temperature induced changes in hydrothermal carbon storage (Stott and Timmermann 2011) require additional evaluation. In that effort it will be important to conduct benthic carbon flux measurements on the flanks of active hydrothermal systems to quantify how these carbon sources contribute to the ocean's carbon budget. It will also be important to investigate trace metal concentrations at sites where there is known sources of SFC-CO 2 such as in the western tropical Pacific (Lupton et al 2008). Earth System Models may also offer opportunities to evaluate how point sources of carbon to the atmosphere such as the upwelling system in the EEP influence the global carbon cycle, including the carbonate system. Such experiments could then be compared with the growing database of observations that include proxies for carbonate ion and pH.

5. Conclusions

In this study we present evidence of increased carbon flux to the upper ocean within the upwelling region of the EEP at the glacial termination. The geochemical measurements support the suggestion that hydrothermal carbon contributed significantly to that increased carbon flux. When combined with results of previous research, it also appears that the increased flux of geologic carbon to the EEP was accompanied by enhanced upwelling strength in one of the ocean's primary conduits for exchange of carbon with the atmosphere. We conclude that the prevailing view that glacial/interglacial pCO 2 variability that characterized the glacial/interglacial cycles was controlled by the storage and subsequent ventilation of respired marine metabolic carbon must be modified to include geological processes that affect the flux of carbon from hydrothermal sources. We further conclude that there is vital importance in learning what regulates the flux of hydrothermal carbon to the ocean. This will require research that explores where and how geologic carbon is stored in the ocean basins and measurements of the actual fluxes of carbon from these sources that can better constrain the marine carbon budget. This research will not only advance knowledge about one of the major scientific challenges in Climate Science but will also provide important constraints on how the marine carbon cycle may be affected by warmer global temperatures in Earth's future.

Acknowledgments