This chapter discusses the role of 14C dating in sea-level research and includes a brief historical overview, an outline of the basic principles of 14C dating, and a discussion of the differences between the radiometric approach and accelerator mass spectrometry (AMS). Sources of error in 14C dating, specifically those relevant to sea-level studies, receive considerable attention. This includes uncertainties associated with isotopic fractionation and reservoir effects. The latter are of particular concern, especially in estuarine environments. Finally, 14C calibration and various widely used programs to convert 14C ages into calendar ages are discussed.
The magnitude of temperature changes in the Early Jurassic are not well known. Clumped isotope measurements can potentially be used to provide better constrains, but unfortunately many of the well-studied sedimentary successions that preserve Lower Jurassic fossils experienced burial temperatures above the limits of preservation of Δ47, which for geological timescales is thought to be between 80–120 °C. Samples from these basins are expected to be partially reordered and yield apparent clumped isotope temperatures that are warmer than original values. Here, we explore whether useful paleoclimate information can be recovered from these samples. We test the hypothesis that relative temperature differences are preserved in partially reordered samples when they experience a common burial history. This was done with the use of reordering models and Δ47 measurements of early Jurassic belemnites from the Aubach section of the SW German Basin, a basin that has a relatively well constrained burial history with maximum burial temperatures above 90 °C. We find that even though partial reordering progressively erases the Δ47 difference between samples, the majority (>50%) of the signal is preserved when samples are buried at temperatures as high of 150 °C for up to 200 Ma. Moreover, the models demonstrate that – regardless of burial conditions – partially reordered samples always preserve minimum records of temperature change across climate events. These inferences are supported by the belemnite Δ47 data that show partially reordered compositions and warming/cooling patterns across the Early Jurassic that closely mimic observations from independent proxies. Model observations are used to interpret a 13 ± 4 °C (95% ci) temperature increase that is observed in the belemnite data across the Early Toarcian. The large magnitude of the temperature excursion is explained as a combination of warming and a change in belemnite habitat before and after the Toarcian Ocean Anoxic Event. Our results demonstrate the usefulness of partially reordered samples and further open the use of this proxy in deep time settings.
Abstract. The evolution of tropical temperature across multiple glacial-interglacial cycles is mostly constrained with marine proxy records, which are associated with considerable uncertainties. Here we present a reconstruction of tropical land temperatures derived from fluid inclusions in stalagmite WR5_B from Whiterock Cave (Gunung Mulu National Park, Northern Borneo). The employed paleothermometer – nucleation-assisted microthermometry – is based on the density of the water trapped in fluid inclusions, i.e., on a well-known thermodynamic parameter and yields highly precise temperature estimates. The record consists of 49 temperature data points spanning a 127 kyr period from 460 to 333 ka including the glacial terminations T-V and T-IV. We find that Borneo temperature tracks Southern hemisphere temperature and atmospheric CO2 concentrations. Deglacial warming is accompanied by relatively dry conditions in Northern Borneo, indicated by pronounced enrichments in calcite δ18Occ and reconstructed drip water δ18Odw values. The amplitude of glacial-interglacial temperature changes amounts to 4.2 ± 0.4 °C (2SEM) between MIS 12 and the MIS 11 interglacial optimum and 4.3 ± 0.4 °C (2SEM) across T-IV. MIS 11 peak temperature was found to be 0.9 ± 0.4 °C warmer than late Holocene temperatures reconstructed for Whiterock Cave, whereas temperatures during MIS 12 and MIS 10 glacial maxima in our record are indistinguishable from those previously reconstructed for the Last Glacial Maximum. Both the present WR5_B record as well as the recently published record from Løland et al. (2022) covering the last glacial Termination exhibit a clear linear correlation with Antarctic temperature anomalies (R2 = 0.89 and 0.97, respectively), with practically identical slopes of the linear regression lines. Depending on the employed Antarctic ΔT reconstruction, Landais et al. (2021) and Jouzel et al. (2007), we found a polar amplification factor of 2.21 ± 0.22 and 2.42 ± 0.23 (95 % CI), respectively.
<p>Ascuns&#259; cave (Romania) is the subject of a monitoring program since 2012. While the cave air temperature was very stable around 7&#176;C for most of the time, it experienced in 2019 a 3&#176;C rise, and remained high until the present.</p><p>We present here &#948;<sup>18</sup>O, &#948;<sup>13</sup>C, and clumped isotope results from calcite farmed at two drip points inside the cave (POM X and POM 2). POM X has a slower drip rate than POM 2 and deposits calcite more continuously. Calcite deposition has been shown to depend on cave air CO<sub>2</sub> concentration, which controls the drip water pH and, further, the calcite saturation index.</p><p>In 2019, &#948;<sup>18</sup>O values at both sites quickly shifted to lower values as a response to the increase in temperature. At POM X, values were situated between approximately -7.2&#8240; and -7.6&#8240; before this transition, whereas in 2019 they shifted to -7.8&#8240; - -8.0&#8240;. At POM 2, where values were generally lower, they shifted from -7.5&#8240; to -7.8&#8240; to -8.0&#8240;.</p><p>Clumped isotope temperature estimates mostly agree, within measurement error, with measured cave temperature. This agreement is notable given that strong offsets are commonly observed in mid-latitude caves, reflecting kinetic fractionation effects. However, intervals with deviations from cave temperature are also observed, suggesting variations in isotopic disequilibrium conditions with time.</p><p>Here we will discuss these isotope changes in relation to cave air temperature and CO<sub>2</sub> concentration, drip water isotope values and elemental chemistry, as well as in relation to drip rates, in order to improve our understanding of calcite precipitation and isotope effects in caves.</p>
Abstract. The southern coast of South Africa displays a highly dynamical climate as it is at the convergence of the Atlantic and Indian oceans, and it is located near the subtropical/temperate zone boundary with seasonal influence of easterlies and westerlies. The region hosts some key archeological sites with records of significant cognitive, technological and social developments. Reconstructions of the state and variability of past climate and environmental conditions around sites of archeological significance can provide crucial context for understanding the evolution of early humans. Here we present a short but high-resolution record of hydroclimate and temperature in South Africa. Our reconstructions are based on trace elements, calcite and fluid inclusion stable isotopes, as well as fluid inclusion microthermometry, from a speleothem collected in Bloukrantz cave, in the De Hoop Nature Reserve in the southern Cape region of South Africa. Our record covers the time period from 48.3 to 45.2 ka during marine isotope stage 3. Both δ18Oc and δ13Cc show strong variability and covary with Sr/Ca. This correlation suggests that the control on these proxies originates from internal cave processes such as prior carbonate precipitation, which we infer to be related to precipitation amount. The hydroclimate indicators furthermore suggest a shift towards overall drier conditions after 46 ka, coincident with cooling in Antarctica and drier conditions in the eastern part of South Africa corresponding to the summer rainfall zone (SRZ). Fluid inclusion-based temperature reconstructions show good agreement between the oxygen isotope and microthermometry methods, and results from the latter display little variation throughout the record, with reconstructed temperatures close to the present-day cave temperature of 17.5 ∘C. Overall, the BL3 speleothem record thus suggests relatively stable temperature from 48.3 to 45.2 ka, whereas precipitation was variable with marked drier episodes on sub-millennial timescales.
The ongoing global warming is characterized by a high latitude amplification effect, with Northern Hemisphere air temperatures increasing significantly faster than the global average. Widely-used paleotemperature proxies suggest that during past warm climate states, there was extreme high-latitude and polar amplified warming, along with flat latitudinal sea surface temperature (SST) gradients. Because these features remain difficult to simulate in climate models for periods like the Miocene, not only model construction, but also absolute values of proxy temperature estimates should be continuously revised. Clumped isotope thermometry is a tool that has the potential to bypass some of the limitations of other proxies, such as reliance on assumptions of past seawater chemistry, and other unknown mechanisms influencing their response to temperature changes. Here we provide the first downcore reconstruction of calcification temperatures from coccolith clumped isotopes (∆47) at northern high latitudes. This record shares trends with alkenone SSTs from the same samples estimated via widely-used calibrations, but suggest an on average ~9 °C colder North Atlantic over the last 16 million years (My). Coccolith ∆47 calcification temperatures agree better than alkenone-derived records with model simulations for the Mid and Late Miocene. If confirmed by additional records, a modest, rather than an extreme northern high latitude warmth, would entail paradigm-changing implications in our understanding of high latitude thermal response to anthropogenic CO2, while implying a need for revision of the present interpretations of currently considered well-validated temperature proxies like alkenone unsaturation ratios.
