ALBERT

Description: The Palaeocene-Eocene Thermal Maximum(1,2) (PETM) was a global warming event that occurred about 56 million years ago, and is commonly thought to have been driven primarily by the destabilization of carbon from surface sedimentary reservoirs such as methane hydrates(3). However, it remains controversial whether such reservoirs were indeed the source of the carbon that drove the warming(1,3-5). Resolving this issue is key to understanding the proximal cause of the warming, and to quantifying the roles of triggers versus feedbacks. Here we present boron isotope data-a proxy for seawater pH-that show that the ocean surface pH was persistently low during the PETM. We combine our pH data with a paired carbon isotope record in an Earth system model in order to reconstruct the unfolding carbon-cycle dynamics during the event(6,7). We find strong evidence for a much larger (more than 10,000 petagrams)-and, on average, isotopically heavier-carbon source than considered previously(8,9). This leads us to identify volcanism associated with the North Atlantic Igneous Province(10,11), rather than carbon from a surface reservoir, as the main driver of the PETM. This finding implies that climate-driven amplification of organic carbon feedbacks probably played only a minor part in driving the event. However, we find that enhanced burial of organic matter seems to have been important in eventually sequestering the released carbon and accelerating the recovery of the Earth system(12).

Description: The circulation and internal structure of the oceans exert a strong influence on Earth’s climate because they control latitudinal heat transport and the segregation of carbon between the atmosphere and the abyss. Circulation change, particularly in the Atlantic Ocean, is widely suggested to have been instrumental in the intensification of Northern Hemisphere glaciation when large ice sheets first developed on North America and Eurasia during the late Pliocene, approximately 2.7 million years ago. Yet the mechanistic link and cause/effect relationship between ocean circulation and glaciation are debated. Here we present new records of North Atlantic Ocean structure using the carbon and neodymium isotopic composition of marine sediments recording deep water for both the Last Glacial to Holocene (35–5 thousand years ago) and the late Pliocene to earliest Pleistocene (3.3–2.4 million years ago). Our data show no secular change. Instead we document major southern-sourced water incursions into the deep North Atlantic during prominent glacials from 2.7 million years ago. Our results suggest that Atlantic circulation acts as a positive feedback rather than as an underlying cause of late Pliocene Northern Hemisphere glaciation. We propose that, once surface Southern Ocean stratification and/or extensive sea-ice cover was established, cold-stage expansions of southern-sourced water such as those documented here enhanced carbon dioxide storage in the deep ocean, helping to increase the amplitude of glacial cycles.

Description: Early diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227/684, 1228/680, 1229/681 and 1230/685) and analysed for petrography, mineralogy, δ13C, δ18O and 87Sr/86Sr values. The results are compared with the chemistry, and δ13C and 87Sr/86Sr values of the associated porewater. Petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate. Dolomite precipitation often pre-dates the formation of framboidal pyrite. Most dolomite layers show 87Sr/86Sr-ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the δ13C values of the dolomite are not in equilibrium with the δ13C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and 87Sr/86Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (〈30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co-occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate–methane interface. It has been hypothesized that microbial ‘hot-spots’, such as the sulphate–methane interface, may act as focused sites of dolomite precipitation. Varying δ13C values from −15‰ to +15‰ for the dolomite are consistent with precipitation at a dynamic sulphate–methane interface, where δ13C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate–methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon-rich sedimentary sequences can provide a useful window into the palaeo-dynamics of the deep biosphere.

Description: The cycling of carbon on Earth exerts a fundamental influence upon the greenhouse gas content of the atmosphere, and hence global climate over millennia. Until recently, ice sheets were viewed as inert components of this cycle and largely disregarded in global models. Research in the past decade has transformed this view, demonstrating the existence of uniquely adapted microbial communities, high rates of biogeochemical/physical weathering in ice sheets and storage and cycling of organic carbon (〉104 Pg C) and nutrients. Here we assess the active role of ice sheets in the global carbon cycle and potential ramifications of enhanced melt and ice discharge in a warming world.

Description: Weddell Sea-derived Antarctic Bottom Water (AABW) is one of the most important deep water masses in the Southern Hemisphere occupying large portions of the deep Southern Ocean (SO) today. While substantial changes in SO-overturning circulation were previously suggested, the state of Weddell Sea AABW export during glacial climates remains poorly understood. Here we report seawater-derived Nd and Pb isotope records that provide evidence for the absence of Weddell Sea-derived AABW in the Atlantic sector of the SO during the last two glacial maxima. Increasing delivery of Antarctic Pb to regions outside the Weddell Sea traced SO frontal displacements during both glacial terminations. The export of Weddell Sea-derived AABW resumed late during glacial terminations, coinciding with the last major atmospheric CO2 rise in the transition to the Holocene and the Eemian. Our new records lend strong support for a previously inferred AABW overturning stagnation event during the peak Eemian interglacial.

Description: The Permian/Triassic boundary approximately 251.9 million years ago marked the most severe environmental crisis identified in the geological record, which dictated the onwards course for the evolution of life. Magmatism from Siberian Traps is thought to have played an important role, but the causational trigger and its feedbacks are yet to be fully understood. Here we present a new boron-isotope-derived seawater pH record from fossil brachiopod shells deposited on the Tethys shelf that demonstrates a substantial decline in seawater pH coeval with the onset of the mass extinction in the latest Permian. Combined with carbon isotope data, our results are integrated in a geochemical model that resolves the carbon cycle dynamics as well as the ocean redox conditions and nitrogen isotope turnover. We find that the initial ocean acidification was intimately linked to a large pulse of carbon degassing from the Siberian sill intrusions. We unravel the consequences of the greenhouse effect on the marine environment, and show how elevated sea surface temperatures, export production and nutrient input driven by increased rates of chemical weathering gave rise to widespread deoxygenation and sporadic sulfide poisoning of the oceans in the earliest Triassic. Our findings enable us to assemble a consistent biogeochemical reconstruction of the mechanisms that resulted in the largest Phanerozoic mass extinction.

Description: The Southern Ocean paleoceanography provides key insights into how iron fertilization and oceanic productivity developed through Pleistocene ice-ages and their role in influencing the carbon cycle. We report a high-resolution record of dust deposition and ocean productivity for the Antarctic Zone, close to the main dust source, Patagonia. Our deep-ocean records cover the last 1.5 Ma, thus doubling that from Antarctic ice-cores. We find a 5 to 15-fold increase in dust deposition during glacials and a 2 to 5-fold increase in biogenic silica deposition, reflecting higher ocean productivity during interglacials. This antiphasing persisted throughout the last 25 glacial cycles. Dust deposition became more pronounced across the Mid-Pleistocene Transition (MPT) in the Southern Hemisphere, with an abrupt shift suggesting more severe glaciations since ~0.9 Ma. Productivity was intermediate pre-MPT, lowest during the MPT and highest since 0.4 Ma. Generally, glacials experienced extended sea-ice cover, reduced bottom-water export and Weddell Gyre dynamics, which helped lower atmospheric CO2 levels.

Description: Antarctica is one of the most vulnerable regions to climate change on Earth and studying the past and present responses of this polar marine ecosystem to environmental change is a matter of urgency. Sedimentary ancient DNA (sedaDNA) analysis can provide such insights into past ecosystem-wide changes. Here we present authenticated (through extensive contamination control and sedaDNA damage analysis) metagenomic marine eukaryote sedaDNA from the Scotia Sea region acquired during IODP Expedition 382. We also provide a marine eukaryote sedaDNA record of ~1 Mio. years and diatom and chlorophyte sedaDNA dating back to ~540 ka (using taxonomic marker genes SSU, LSU, psbO). We find evidence of warm phases being associated with high relative diatom abundance, and a marked transition from diatoms comprising 〈10% of all eukaryotes prior to ~14.5 ka, to ~50% after this time, i.e., following Meltwater Pulse 1A, alongside a composition change from sea-ice to open-ocean species. Our study demonstrates that sedaDNA tools can be expanded to hundreds of thousands of years, opening the pathway to the study of ecosystem-wide marine shifts and paleo-productivity phases throughout multiple glacial-interglacial cycles.

Description: During the last deglaciation substantial volumes of meltwater from the decaying Laurentide Ice Sheet were supplied to the Arctic, Gulf of Mexico and North Atlantic along different drainage routes, sometimes as catastrophic flood events. These events are suggested to have impacted global climate, for example initiating the Younger Dryas cold period. Here we analyze the authigenic Pb isotopic composition of sediments in front of the Arctic Mackenzie Delta, a sensitive tracer for elevated freshwater runoff of the retreating Laurentide Ice Sheet. Our data reveal continuous meltwater supply to the Arctic along the Mackenzie River since the onset of the Bølling–Allerød. The strongest Lake Agassiz outflow event is observed at the end of the Bølling–Allerød close to the onset of the Younger Dryas. In context of deglacial North American runoff records from the southern and eastern outlets, our findings provide a detailed reconstruction of the deglacial drainage chronology of the disintegrating Laurentide Ice Sheet.

Description: Dissolved lead (Pb) is mainly supplied to the oceans by physical and chemical weathering on the continents. The short residence time of Pb in seawater on the order of only a few decades makes its isotopic compositions an excellent tracer for local continental inputs. Lead was found to be incongruently released during early chemical weathering on the continents (Erel et al., 1994), often generating a more radiogenic runoff signal compared to the bulk rock compositions (Gutjahr et al., 2009; Kurzweil et al., 2010; Crocket et al., 2012; Crocket et al., 2013). In addition, the presence of abundant ice-rafted detrital material (IRD) may also release a highly radiogenic signature in high latitude settings (Kurzweil et al., 2010; Crocket et al., 2012). In the (sub-)Antarctic marine environment, authigenic Pb isotope records from core top sediments offer the possibility of assessing spatial seawater Pb isotopic variability of subglacial Antarctic runoff. Furthermore, palaeo-seawater Pb isotope records extracted from authigenic Fe-Mn oxyhydroxides will likely record periods of enhanced iceberg calving, freshwater input, and/or associated circulation changes. Since the leaching method for extracting authigenic Pb from Antarctic proximal bulk sediments has not been studied to date, we firstly evaluated and refined existing reductive leaching methods (Gutjahr et al., 2007; Blaser et al., 2016;) for efficient and reliable chemical extraction of bottom seawater Pb isotope signals from Weddell Sea and Southern Ocean core top sediment samples. We investigated the effects of (i) the MgCl2 pre-treatment, (ii) the effectiveness of chelates as well as (iii) exposure time of sediments to reducing reagents on the Pb isotopic signals. Chelate EDTA shows stronger complexation ability to Pb than DTPA and can significantly prevent Pb from readsorption back onto sediment surfaces during leaching as described in previous studies (Gutjahr et al., 2007). We also found that leaching without extended (〉20 min) shaking, hence only agitating sediments for less than a minute on a vortex mixer to help sediment disperse into leaching solution, can extract quantities of Pb as extracted with via leaching for 20 minutes in a shaker. Using this short-term “vortexing” method, reproducible and in most cases accurate isotopic ratios identical or close to seawater signals can be obtained. Therefore we suggest using the vortexing method with EDTA and without MgCl2 pre-treatment to recover authigenic Pb from Antarctic ice shelf-proximal bulk sediments. Employing this new method, we present Pb isotope records from 90 core top sediment samples from the Weddell Sea and the Atlantic sector of Southern Ocean covering ~4000 km of the Weddell Sea Antarctic continental margin. Furthermore, first results are presented from IODP Site 1094 delineating the authigenic Pb isotopic evolution over the past 140 ka tracing Antarctic ice sheet dynamics and Southern Ocean circulation. References: Blaser, P., Lippold, J., Gutjahr, M., Frank, N., Link, J.M., Frank, M., 2016. Extracting foraminiferal seawater Nd isotope signatures from bulk deep sea sediment by chemical leaching. Chemical Geology 439, 189-204. Crocket, K.C., Foster, G.L., Vance, D., Richards, D.A., Tranter, M., 2013. A Pb isotope tracer of ocean-ice sheet interaction: the record from the NE Atlantic during the Last Glacial/Interglacial cycle. Quaternary Science Reviews 82, 133-144. Crocket, K.C., Vance, D., Foster, G.L., Richards, D.A., Tranter, M., 2012. Continental weathering fluxes during the last glacial/interglacial cycle: insights from the marine sedimentary Pb isotope record at Orphan Knoll, NW Atlantic. Quaternary Science Reviews 38, 89-99. Erel, Y., Harlavan, Y., Blum, J.D., 1994. Lead isotope systematics of granitoid weathering. Geochimica et Cosmochimica Acta 58, 5299-5306. Gutjahr, M., Frank, M., Halliday, A.N., Keigwin, L.D., 2009. Retreat of the Laurentide ice sheet tracked by the isotopic composition of Pb in western North Atlantic seawater during termination 1. Earth and Planetary Science Letters 286, 546-555. Gutjahr, M., Frank, M., Stirling, C.H., Klemm, V., Flierdt, T., Halliday, A.N., 2007. Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments. Chemical Geology 242, 351-370. Kurzweil, F., Gutjahr, M., Vance, D., Keigwin, L., 2010. Authigenic Pb isotopes from the Laurentian Fan: Changes in chemical weathering and patterns of North American freshwater runoff during the last deglaciation. Earth and Planetary Science Letters 299, 458-465.