The Boring Millions? Vegetation, Atmospheric CO2 , and Climate Revolutions of the Late Miocene

During the late Miocene (11-5 Ma), global ice volume and deep ocean temperatures appear to be relatively unchanging. These “boring millions” suggest stasis of the climate system with the expectation of only moderate global changes in climate, CO2 and vegetation. However, during this time tropical ecosystems underwent profound changes and surface ocean temperatures declined dramatically. When did these changes occur, what drove them, and what role if any did atmospheric carbon dioxide levels play? I will address these questions through new observations of the onset, pace and geographic extent of vegetation transformations and hydrologic changes reconstructed from molecular biomarkers. I will then examine the role that atmospheric CO2 levels and other factors may have played in these transformations

Uncertainty in paleohydrologic reconstructions from molecular δD values

Compound-specific δD measurements can be used for quantitative estimation of source water δD values, a useful tracer for paleohydrologic changes. Such estimates have quantifiable levels of uncertainty that are often miscalculated, resulting in inaccurate error reporting in the scientific literature that can impact paleohydrologic interpretations. Here, we summarize the uncertainties inherent to molecular δD measurements and the quantification of source water δD values, and discuss the assumptions involved when omitting various sources of uncertainty. Using standard protocols from measurement science, we derive the equations necessary to quantify these various sources of uncertainty. We show that analytical uncertainty is usually improperly estimated and that after apparent fractionation between δD values of source water and molecule, normalization of data to the VSMOW scale introduces the largest amount of uncertainty. Lastly, to facilitate systematic error reporting we provide an Uncertainty Calculator spreadsheet to conveniently calculate uncertainty in δD measurements

Organic thermal maturity as a proxy for frictional fault heating: Experimental constraints on methylphenanthrene kinetics at earthquake timescales

Biomarker thermal maturity is widely used to study burial heating of sediments over millions of years. Heating over short timescales such as during earthquakes should also result in measurable increases in biomarker thermal maturity. However, the sensitivity of biomarker thermal maturity reactions to short, higher-temperature heating has not been established. We report on hydrous pyrolysis experiments that determine the kinetic parameters of methylphenanthrene maturation at timescales and temperatures relevant to earthquake heating. Samples of Woodford Shale were heated at temperatures up to 343 °C over 15–150 min. The thermal maturity of the samples as measured by the methylphenanthrene index-1 (MPI-1) increased with heating time and temperature. We find that MPI-1 increases with time and temperature consistent with a first-order kinetic model and Arrhenius temperature relationship. Over the timescales tested here, MPI-1 is strongly affected by maximum temperature and less sensitive to heating duration. Production of new phenanthrene isomers and expulsion of a liquid pyrolyzate also occurred. Differential expulsion of methylphenanthrene isomers affected the apparent maturity of the rock at lower temperatures and may need to be considered for organic-rich fault rocks. Our results demonstrate that the overall MPI-1 reaction extent in both the rock and pyrolyzate are a useful measure of thermal maturity and reflect temperature history during rapid heating

Earthquake slip surfaces identified by biomarker thermal maturity within the 2011 Tohoku-Oki earthquake fault zone.

Extreme slip at shallow depths on subduction zone faults is a primary contributor to tsunami generation by earthquakes. Improving earthquake and tsunami risk assessment requires understanding the material and structural conditions that favor earthquake propagation to the trench. We use new biomarker thermal maturity indicators to identify seismic faults in drill core recovered from the Japan Trench subduction zone, which hosted 50 m of shallow slip during the Mw9.1 2011 Tohoku-Oki earthquake. Our results show that multiple faults have hosted earthquakes with displacement ≥ 10 m, and each could have hosted many great earthquakes, illustrating an extensive history of great earthquake seismicity that caused large shallow slip. We find that lithologic contrasts in frictional properties do not necessarily determine the likelihood of large shallow slip or seismic hazard

Relationship between individual chamber and whole shell Mg/Ca ratios in Trilobatus sacculifer and implications for individual foraminifera palaeoceanographic reconstructions

Precisely targeted measurements of trace elements using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) reveal inter-chamber heterogeneities in specimens of the planktic foraminifer Trilobatus (Globigerinoides) sacculifer. We find that Mg/Ca ratios in the final growth chamber are generally lower compared to previous growth chambers, but final chamber Mg/Ca is elevated in one of thirteen sample intervals. Differences in distributions of Mg/Ca values from separate growth chambers are observed, occurring most often at lower Mg/Ca values, suggesting that single-chamber measurements may not be reflective of the specimen’s integrated Mg/Ca. We compared LA-ICPMS Mg/Ca values to paired, same-individual Mg/Ca measured via inductively coupled plasma optical emission spectrometry (ICP-OES) to assess their correspondence. Paired LA-ICPMS and ICP-OES Mg/Ca show a maximum correlation coefficient of R = 0.92 (p \u3c 0.05) achieved by applying a weighted average of the last and penultimate growth chambers. Population distributions of paired Mg/Ca values are identical under this weighting. These findings demonstrate that multi-chamber LA-ICPMS measurements can approximate entire specimen Mg/Ca, and is thus representative of the integrated conditions experienced during the specimen’s lifespan. This correspondence between LA-ICPMS and ICP-OES data links these methods and demonstrates that both generate Mg/Ca values suitable for individual foraminifera palaeoceanographic reconstructions

Relationship between individual chamber and whole shell Mg/Ca ratios in Trilobatus sacculifer and implications for individual foraminifera palaeoceanographic reconstructions

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Rustic, G. T., Polissar, P. J., Ravelo, A. C., & DeMenocal, P. Relationship between individual chamber and whole shell Mg/Ca ratios in Trilobatus sacculifer and implications for individual foraminifera palaeoceanographic reconstructions. Scientific Reports, 11(1), (2021): 463, https://doi.org/10.1038/s41598-020-80673-8.Precisely targeted measurements of trace elements using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) reveal inter-chamber heterogeneities in specimens of the planktic foraminifer Trilobatus (Globigerinoides) sacculifer. We find that Mg/Ca ratios in the final growth chamber are generally lower compared to previous growth chambers, but final chamber Mg/Ca is elevated in one of thirteen sample intervals. Differences in distributions of Mg/Ca values from separate growth chambers are observed, occurring most often at lower Mg/Ca values, suggesting that single-chamber measurements may not be reflective of the specimen’s integrated Mg/Ca. We compared LA-ICPMS Mg/Ca values to paired, same-individual Mg/Ca measured via inductively coupled plasma optical emission spectrometry (ICP-OES) to assess their correspondence. Paired LA-ICPMS and ICP-OES Mg/Ca show a maximum correlation coefficient of R = 0.92 (p < 0.05) achieved by applying a weighted average of the last and penultimate growth chambers. Population distributions of paired Mg/Ca values are identical under this weighting. These findings demonstrate that multi-chamber LA-ICPMS measurements can approximate entire specimen Mg/Ca, and is thus representative of the integrated conditions experienced during the specimen’s lifespan. This correspondence between LA-ICPMS and ICP-OES data links these methods and demonstrates that both generate Mg/Ca values suitable for individual foraminifera palaeoceanographic reconstructions.This research was funded by National Science Foundation Grants OCE 1401649 and OCE 1405178, the Lamont-Doherty Earth Observatory Climate Center, and the Columbia Center for Climate and Life

Constraints on the salinity–oxygen isotope relationship in the central tropical Pacific Ocean

Uncertainties surround the relationship between salinity and the stable isotopic composition of seawater, largely due to a dearth of modern seawater isotope data. Here we report 191 new, paired measurements of salinity and seawater oxygen isotopes (δ¹⁸O_sw) taken from the central tropical Pacific in May 2012, from the surface to 4600 m depth. We observe significant correlations between δ¹⁸O_sw and salinity across the study region, with slopes ranging from 0.23 to 0.31‰/psu for the mixed layer, and 0.35–0.42‰/psu for waters between the mixed layer and 500 m depth. When considering δ¹⁸O_sw–salinity across averages of individual water masses in the region, slopes range from 0.21 to 0.40‰/psu, albeit with appreciable scatter. Surface salinity and δ¹⁸O_sw data corresponding to the North Equatorial Countercurrent are significantly higher than previously observed, which we attribute to a weak westerly current and dry conditions in the region during the May 2012 cruise. Subsurface (80–500 m) salinity values from 2012 are significantly lower than corresponding values from pre-existing regional data, highlighting a different latitudinal sampling distribution, while subsurface δ¹⁸O_sw is not significantly different. Thus, in May 2012, δ¹⁸O_sw in this region could not be used to distinguish between subsurface water masses of different salinities. Unlike other regions where the surface ‘freshwater endmember’ is close to the δ¹⁸O value of regional precipitation, the freshwater endmember implied by our dataset (− 10.38‰) is consistent with a strong evaporative influence. Paired δ¹⁸O–δD values of precipitation and surface seawaters have similar slopes (5.0, 5.1), and relatively low intercepts (1.4, 0.8) indicating isotopic variability in both reservoirs is also partly controlled by evaporation

Proglacial lake sediment records reveal Holocene climate changes in the Venezuelan Andes

Lake sediment records from the Cordillera de Mérida in the northern Venezuelan Andes document the history of local glacial variability and climate changes during the Holocene (∼12 ka to the present). The valleys that contain these lakes have similar bedrock compositions and hypsometries, but have different headwall elevations and aspects, which makes them useful for investigating the magnitude of past glaciations. There was widespread glacial retreat in the Venezuelan Andes during the early Holocene, after which most watersheds remained ice free, and thus far only valleys with headwalls higher than ∼4400 m asl contain evidence of glaciation during the last ∼10 ka. There was a pronounced shift in sediment composition for the Montos (headwall: ∼4750 m asl) and Los Anteojos (headwall: ∼4400 m asl) records during the middle Holocene from ∼8.0 to 7.7 ka when conditions appear to have become ice free and drier. There is tentative evidence that the glacier in the Mucubají valley (headwall: ∼4609 m asl) advanced from ∼8.1 to 6.6 ka and then retreated during the latter stages of the middle Holocene. Clastic sediment accumulation in other nearby lake basins was either low or decreased throughout most of the middle Holocene as watersheds stabilized under warmer and/or drier conditions. In the Montos record, there was another major shift in sediment composition that occurred from ∼6.5 to 5.7 ka, similar to other regional records that suggest conditions were drier during this period. Overall, the late Holocene was a period of warmer and wetter conditions with ice extent at a minimum in the northern tropical Andes. There were also punctuated decadal to multi-centennial periods of higher clastic sediment accumulation during the last ∼4 ka, likely in response to periods of cooling and/or local precipitation changes. In watersheds with headwalls above 4600 m asl, there is evidence of glacial advances during the Little Ice Age (∼0.6–0.1 ka). The pattern of glacial variability is generally similar in both the northern and southern tropics during the Little Ice Age, suggesting that ice margins in both regions were responding to colder and wetter conditions during the latest Holocene. The observed pattern of Holocene climate variability in the Venezuelan Andes cannot be explained by insolation forcing alone, and tropical ocean influences were likely associated with the observed glacial and lake level changes

Midlatitude Temperature Variations in the Oligocene to Early Miocene

[EN]Antarctic ice sheet margin extent and the sensitivity of benthic δ18O to orbital forcing have varied on million-year timescales during the Oligocene to Early Miocene. However, few sea surface temperature (SST) records for this time interval exist to evaluate links between polar processes and mean temperature outside polar regions. Here, we present a new record of SST for the time interval 30 to 17 Ma derived from the long-chain alkenone unsaturation ratio ( urn:x-wiley:25724517:media:palo20775:palo20775-math-0001) at Integrated Ocean Drilling Program Site 1406A in the midlatitude North Atlantic. Results confirm that warm temperatures from 24°C to over 30°C prevailed in midlatitudes in this time and suggest a transition from colder early-middle Oligocene to warmer average conditions after 24.5 Ma. The global significance of this transition is highlighted by the coincidence with changes in the dominance from marine- to terrestrial-terminating ice sheets in the Ross Sea around Antarctica. The longest continuous section of the record (20.6 to 26.6 Ma) contains multiple 2 million-year cycles in SST, potentially paced by long obliquity modulation. Complex and temporally varying relationships are observed between North Atlantic SST and benthic δ18O in paired samples; significant covariation is only observed around the Oligocene-Miocene transition, coincident with a lower average marine ice extent. These North Atlantic urn:x-wiley:25724517:media:palo20775:palo20775-math-0002 temperature records provide a new context in which to examine the stability of climate and the Antarctic ice sheet during the Oligocene and early Miocene

Upregulation of phytoplankton carbon concentrating mechanisms during low CO2 glacial periods and implications for the phytoplankton pCO2 proxy

Unidad de excelencia María de Maeztu MdM-2015-0552Published alkenone εp records spanning known glacial pCO2 cycles show considerably less variability than predicted by the diffusive model for cellular carbon acquisition and isotope fractionation. We suggest this pattern is consistent with a systematic cellular enhancement of the carbon supply to photosynthesis via carbon concentrating mechanisms under the case of carbon limitation during low pCO2 glacial time periods, an effect also manifest under carbon limitation in experimental cultures of coccolithophores as well as diatoms. While the low-amplitude εp signal over glacial pCO2 cycles has led some to question the reliability of εp for reconstructing long-term pCO2, the [CO2]aq in the tropical oceans during glacial pCO2 minima represents the most extreme low CO2 conditions likely experienced by phytoplankton in the Cenozoic, and the strongest upregulation of carbon concentrating mechanisms. Using a statistical multilinear regression model, we quantitatively parse out the factors (namely light, growth rate, and [CO2]aq), that contribute to variation in εp in alkenone-producing algae, which confirms a much smaller dependence of εp on [CO2]aq in the low [CO2]aq range, than inferred from the hyperbolic form of the diffusive model. Application of the new statistical model to two published tropical εp records spanning the late Neogene produces much more dynamic pCO2 estimates than the conventional diffusive model and reveals a significant pCO2 decline over the last 15 Ma, which is broadly consistent with recent results from boron isotopes of foraminifera. The stable isotopic fractionation between coccolith calcite and seawater dissolved inorganic carbon (here Δcoccolith-DIC) also shows systematic variations over glacial-interglacial cycles which may, following future experimental constraints, help estimate the degree of upregulation of parts of the algal carbon concentrating mechanism over glacial cycles