By the year 2400, it is predicted that humans will have released about 5,000 gigatonnes of carbon to the atmosphere since the start of the industrial revolution if fossil-fuel emissions continue ...unabated and carbon-sequestration efforts remain at current levels. But now past episodes of greenhouse warming are providing insight into the coupling of climate and the carbon cycle and thus may help to predict the consequences of unabated carbon emissions in the future.
The start of the Palaeocene/Eocene thermal maximum-a period of exceptional global warming about 55 million years ago-is marked by a prominent negative carbon isotope excursion that reflects a massive ...input of 13C-depleted ('light') carbon to the ocean-atmosphere system. It is often assumed that this carbon injection initiated the rapid increase in global surface temperatures and environmental change that characterize the climate perturbation, but the exact sequence of events remains uncertain. Here we present chemical and biotic records of environmental change across the Palaeocene/Eocene boundary from two sediment sections in New Jersey that have high sediment accumulation rates. We show that the onsets of environmental change (as recorded by the abundant occurrence ('acme') of the dinoflagellate cyst Apectodinium) and of surface-ocean warming (as evidenced by the palaeothermometer TEX86) preceded the light carbon injection by several thousand years. The onset of the Apectodinium acme also precedes the carbon isotope excursion in sections from the southwest Pacific Ocean and the North Sea, indicating that the early onset of environmental change was not confined to the New Jersey shelf. The lag of ∼3,000 years between the onset of warming in New Jersey shelf waters and the carbon isotope excursion is consistent with the hypothesis that bottom water warming caused the injection of 13C-depleted carbon by triggering the dissociation of submarine methane hydrates, but the cause of the early warming remains uncertain.
Massive amounts of
13C-depleted carbon rapidly entered the ocean more than once during the Early Paleogene, providing a geological framework for understanding future perturbations in carbon cycling, ...including ocean acidification. To assess the number of events and their impact on deep-sea carbonate accumulation, we investigated a 42
m thick unit of Upper Paleocene–Lower Eocene carbonate ooze, which was deposited on a subsiding flank of the East Pacific Rise. Age control was established using calcareous nannofossils and planktonic foraminifera, as well as stable carbon isotopes of bulk carbonate. Carbonate content, foraminiferal test fragmentation, and planktonic/benthic foraminiferal ratio were measured to ascertain changes in carbonate dissolution. Based on these analyses, carbonate preservation generally increased from the late Paleocene (55.4
Ma) through the early Eocene (51.4
Ma), after which it became poor to negligible. This trend was punctuated by three (and probably four) short-term intervals characterized by carbonate dissolution and negative δ
13C excursions. These horizons almost assuredly correspond to the PETM (~
55.5
Ma), H1/ETM-2 (~
53.7
Ma), I1 (~
53.2
Ma), and K/X (~
52.5
Ma) events. Carbonate preservation also increased within 200
kyr after two and perhaps all four intervals. We suggest the lysocline and calcite compensation depth (CCD) generally deepened between 55.4 and 51.4
Ma but shoaled and subsequently overcompensated during and after three and likely four intervals of rapid and massive carbon injection. Oxygen isotope data further suggests these intervals were times of anomalous warmth.
The Palaeocene/Eocene thermal maximum represents a period of rapid, extreme global warming ∼55 million years ago, superimposed on an already warm world. This warming is associated with a severe ...shoaling of the ocean calcite compensation depth and a >2.5 per mil negative carbon isotope excursion in marine and soil carbonates. Together these observations indicate a massive release of 13C-depleted carbon and greenhouse-gas-induced warming. Recently, sediments were recovered from the central Arctic Ocean, providing the first opportunity to evaluate the environmental response at the North Pole at this time. Here we present stable hydrogen and carbon isotope measurements of terrestrial-plant- and aquatic-derived n-alkanes that record changes in hydrology, including surface water salinity and precipitation, and the global carbon cycle. Hydrogen isotope records are interpreted as documenting decreased rainout during moisture transport from lower latitudes and increased moisture delivery to the Arctic at the onset of the Palaeocene/Eocene thermal maximum, consistent with predictions of poleward storm track migrations during global warming. The terrestrial-plant carbon isotope excursion (about -4.5 to -6 per mil) is substantially larger than those of marine carbonates. Previously, this offset was explained by the physiological response of plants to increases in surface humidity. But this mechanism is not an effective explanation in this wet Arctic setting, leading us to hypothesize that the true magnitude of the excursion-and associated carbon input-was greater than originally surmised. Greater carbon release and strong hydrological cycle feedbacks may help explain the maintenance of this unprecedented warmth.
The history of the Arctic Ocean during the Cenozoic era (0-65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m ...of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm 'greenhouse' world, during the late Palaeocene and early Eocene epochs, to a colder 'icehouse' world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent approximately 14 Myr, we find sedimentation rates of 1-2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (approximately 3.2 Myr ago) and East Antarctic ice (approximately 14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (approximately 45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at approximately 49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (approximately 55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.
To improve the understanding and utility of bulk carbonate stable carbon and oxygen isotope measurements, we examine sediment from cores in the eastern equatorial Pacific that span the last 8 Ma. We ...measured δ13C and δ18O in 791 samples from Integrated Ocean Drilling Program Site U1338 and Deep Sea Drilling Project Site 573, both located close to the Pacific equator. In 100 samples, we measured δ13C and δ18O on isolated <63 µm and <38 µm fractions, which concentrates calcareous nannofossil carbonate and progressively excludes foraminiferal carbonate. Bulk carbonate δ13C and δ18O records are similar to published records from other sites drilled near the equator and seem to reflect mixed layer conditions, albeit with some important caveats involving the precipitation of calcite by coccolithophores. The comparatively lower δ13C and δ18O of the <63 µm and <38 µm fractions in sediments younger than 4.4 Ma is attributed to an increase in deep‐dwelling planktic foraminifera material in bulk carbonate, shifting the bulk isotopic signals toward higher values. Bulk carbonate δ13C is similar over 2500 km along the Pacific equator, suggesting covarying concentrations and δ13C of dissolved inorganic carbon within surface waters since 8 Ma. Greater bulk sediment δ13C and δ18O, higher sedimentation rates, and low content of coarse material suggest intensified wind‐driven upwelling and enhanced primary productivity along the Pacific equator between 8.0 and 4.4 Ma, although a full understanding of bulk carbonate records will require extensive future work.
Key Points
Bulk carbonate δ13C and δ18O data primarily record mixed layer conditions
Coccolithophore biogeochemistry impacts bulk stable isotope records
Data from Sites U1338 and 573 suggest enhanced upwelling between 8 Ma and 4 Ma
The Pacific plate circuit went through a complex reorganization during the early to middle Eocene, approximately coinciding with the onset of subduction along the western Pacific margin. However, the ...timing and dynamics of this change in the southwest Pacific and evolution of subduction beneath the Tonga‐Kermadec Arc are not fully resolved. We present magneto‐biostratigraphic data from an early to middle Eocene sedimentary section exposed in the Koumac‐Gomen area, New Caledonia, which is an emerged portion of the Norfolk Ridge. The 260 m‐thick succession contains a transition from pelagic micrite to terrigenous‐rich calciturbidite that is observed regionally in New Caledonia and which is interpreted to represent a shift from sedimentation on a stable submarine plateau to slope formation developed under a convergent tectonic regime. The stratigraphic contact between pelagic micrite and overlying calciturbidite is not exposed, but our magnetic polarity‐based chronology constrains the age of transition to 46–44 Ma, in agreement with the 45.3 Ma age recently obtained from the Noumea area in southern New Caledonia. We integrate records from New Caledonia with recent magnetostratigraphic data from South Island, New Zealand, where marked variations in terrigenous input occurred during the early and middle Eocene. Synchronous sedimentary changes in the southwest Pacific occurred at the same time as onset of rapid seafloor spreading south of Australia and New Zealand. We infer that the underlying cause of stratigraphic change was inception of slip at a new configuration of the Australia‐Pacific plate boundary, which evolved into the Tonga‐Kermadec subduction system.
Plain language summary
New Caledonia and New Zealand expose sedimentary rocks that hold the key for understanding regional tectonic evolution. Using rock magnetism and micropaleontology, we determine the age of sedimentary records in New Caledonia. A change in sedimentation style indicates the onset of tectonic activity about 45 million years ago, which is the same time that significant changes in depositional style and rate are recorded by sediments in New Zealand, and changes in spreading rate are recorded by magnetic anomalies south of Australia and New Zealand. We interpret our evidence as recording inception of a new plate boundary in the southwest Pacific that ultimately resulted in development of the southwestern portion of the Pacific Ring of Fire.
Key Points
We present a new integrated magneto‐biostratigraphic age model for Eocene sedimentary rocks in New Caledonia
The regional onset of sediment gravity flows at ~46–44 Ma corresponds to the inception of rapid Australia‐Pacific plate motion
Northern Zealandia sedimentary change during the middle Eocene was likely related to initiation of the Tonga‐Kermadec subduction
Anthropogenic warming of the oceans can release methane (CH4) currently stored in sediments as gas hydrates. This CH4 will be oxidized to CO2, thus increasing the acidification of the oceans. We ...employ a biogeochemical model of the multimillennial carbon cycle to determine the evolution of the oceanic dissolved carbonate system over the next 13 kyr in response to CO2 from gas hydrates, combined with a reasonable scenario for long‐term anthropogenic CO2 emissions. Hydrate‐derived CO2 will appreciably delay the neutralization of ocean acidity and the return to preindustrial‐like conditions. This finding is the same with CH4 release and oxidation in either the deep ocean or the atmosphere. A change in CaCO3 export, coupled to CH4 release, would intensify the transient rise of the carbonate compensation depth, without producing any changes to the long‐term evolution of the carbonate system. Overall, gas hydrate destabilization implies a moderate additional perturbation to the carbonate system of the Anthropocene oceans.
Key Points
Methane hydrate hydrates will dissociate with global warming
This methane will oxidize to CO2 and further acidify the oceans
This will appreciably prolong anthropogenic ocean acidification
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