Geology topics

Bradley R. Markle

Publications and source records attributed to Bradley R. Markle.

3 recordsLinked to original sources

USGS2026

On the fidelity of oxygen isotope-based sea level reconstructions: A case study of Marine Isotope Stage 3

Although foraminiferal oxygen isotope records (δ 18 O F ) have long been used to develop time-continuous estimates of past global ice volume, growing evidence indicates that the ice volume and temperature contributions to δ 18 O F diverge considerably over the last glacial cycle. Across Marine Isotope Stage 3 (MIS 3; 57 to 29 ka), estimates of peak global sea level based on δ 18 O F are ∼30 m lower than reconstructions based on recent glacial geology, geologic sea-level records, and glacial isostatic adjustment (GIA) modeling approaches. Here we address this mismatch by quantifying uncertainties in converting δ 18 O F to global sea level during MIS 3. We review the geochemical tools to assess the contributions of ocean temperature, diagenesis, and the isotopic composition of ice sheets to δ 18 O F records across MIS 3. We show that recent advances in our understanding of these and other factors that contribute uncertainty (e.g., global sea level during the Last Glacial Maximum) help to explain the MIS 3 gap between global sea level estimates based on δ 18 O F and GIA analyses of geologic sea-level records. We find that the accuracy and precision of MIS 3 ocean temperature reconstructions is likely a key contributor, and various MIS 3 temperature reconstructions can reduce the “gap” by 30%–70% (∼10–20 m). Future improvements in the precision of temperature reconstructions will better constrain the ice volume component of δ 18 O F records. Ultimately, assessing the fidelity of δ 18 O F as a proxy for global ice volume is key to disentangling the feedbacks between climate and ice sheets over the Pleistocene.

Paleoceanography and Paleoclimatology↗

USGS2015

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives 1 . Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa 2 , 3 , suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw 4 , 5 , 6 . Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events 7 , 8 , 9 . Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision 2 , 3 , 10 . Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2 σ ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics.

Nature↗

USGS2013

Onset of deglacial warming in West Antarctica driven by local orbital forcing

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate. Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently. Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere associated with an abrupt decrease in Atlantic meridional overturning circulation. However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

Nature↗