SUMMER PRAETORIUS, PhD
Understanding the causes and effects of rapid climate change in the past to identify systems that may be susceptible to abrupt thresholds in response to future climate change.
The oceans are the great integrator of Earth's climate, and also the source of short-term and long-term climate variability. Ocean sediments act as a tape recorder of climate fluctuations in the past, providing a way to link ocean dynamics with regional and global climate changes.
Most of my research has involved paleoclimate reconstructions using various geochemical proxies from deep sea sediment cores, such as stable carbon and oxygen isotopes in planktonic and benthic foraminifera, the alkenone UK'37 paleotemperature index, radiocarbon for chronology and water mass ventilation rate, grain size analysis for measuring changes in deep water flow rate, tephra geochemistry, and foraminiferal faunal assemblages. These proxies provide a means to reconstruct regional changes in ocean conditions in order to understand how the ocean is both responding to and driving global climate change.
The role of the North Pacific in driving rapid climate changes of the past has received much less
attention than other oceanic regions, such as the North Atlantic and Southern Ocean. I have been
working to create high- resolution paleoceanographic records in the North Pacific to better understand how rapid climate changes in this region may manifest in response to radiative forcing and dynamic interactions with other oceanic regions.
Most recently, I have been using global climate models to address how changes in ocean heat patterns affect regional climate.
Recent research topics are summarized below.
A number of early warning signals have been identified to precede tipping points in various complex systems, such as increased variance, autocorrelation, and spatial correlation. High connectivity is predicted to make complex systems more susceptible to threshold collapse, as local perturbations can cascade through the larger system, slowly eroding the resilience until a small perturbation propels the system into an alternate state.
The climate system exhibits a number of abrupt transitions in the past. Understanding the physical mechanisms that triggered these events, as well as determining whether they exhibited early warning signals prior to the transitions is critical for assessing the likelihood of similar events occuring in the future.
In a recent paper, we developed a new high resolution oxygen istope record from the Gulf of Alaska and compared this with the Greenland oxygen isotope record to evaluate changing climate patterns between the subpolar North Pacific and North Atlantic regions throughout the past 18,000 years. This study revealed switches between synchronized and seesaw-like climate patterns between these regions through time. We found that the records were highly synchronized during the most abrupt climate transitions of the past 18,000 years, consistent with theory predicting enhanced spatial organization prior to tipping points. We suggest that synchronized ocean heat transport in the North Pacific and North Atlantic may have led to amplified temperature swings in the Northern Hemisphere, such as occurred during the rapid warmings into the Bolling and Holocene periods.
S.K. Praetorius & A.C. Mix (2014). Synchronization of North Pacific and Greenland climate preceded abrupt deglacial warming. Science 345:444-448.
S.K. Praetorius & A.C. Mix (2016). Did synchronized ocean warming in the north
pacific and north Atlantic trigger a deglacial tipping point in the northern Hemisphere? PAGES 24:10-11.
The North Pacific hosts the world’s most expansive oxygen minimum zone, making many coastal regions along the Pacific rim particularly sensitive to hypoxic conditions, in which oxygen concentrations fall below levels of tolerance for marine life and alter the cycling of nutrients and carbon into the deep ocean.
Episodes of widespread hypoxia across the North Pacific margins have been observed in the not-so-distant past. These episodes of hypoxia appear to have started abruptly, but persisted for millennia, implying threshold-like dynamics that tipped these regions into hypoxia and sustained conditions through amplifying feedbacks.
Together, with others researchers from Oregon State University, we developed a high-resolution compilation of proxy data from a marine sediment core in the Gulf of Alaska to examine the precise sequence of oceanic changes leading up to the onset of hypoxia. These records reveal a strong link between a rapid rise in sea surface temperature and the onset of hypoxia, which also coincided with an increase in the amount of marine plankton sinking to the sea floor. We infer that ocean warming led to a modest expansion of the oxygen minimum zone through reduced oxygen solubility, and was amplified by a biological response favoring diatom growth and export, further consuming subsurface oxygen through respiration of sinking organic matter. We hypothesize that iron release from hypoxic sediments may have been a mechanism to trigger/sustain diatom growth, together with warm surface conditions.
These results imply that low-oxygen ocean regions may be more sensitive to future warming than currently predicted, if a similar export productivity feedback (dominated by diatoms) were to occur in the future.
S.K. Praetorius, A.C. Mix, M.H. Walczak, M.D. Wolhowe, J.A. Addison, F.G. Prahl. (2015) North Pacific deglacial hypoxic events linked to abrupt ocean warming. Nature 527:362-366
It is well documented from historical events that volcanic eruptions can cause rapid climate fluctuations on short time scales (1-10 years). However, it is less clear how clusters of volcanic events may impact climate on longer time scales (10-1000 yrs), in addition to how volcanic frequency is modulated by long-term climate change.
During the transition from the last ice age into the modern interglacial period, a dramatic increase in volcanism is observed in the Northern Hemisphere. This increase in eruptive frequency has been proposed as a response to the depressurization effects of waning ice sheets overlying volcanic terrain.
By looking at tephra layers preserved in a marine sediment core near Mt. Edgecumbe in Southeast Alaska, we were able to reconstruct the precise timing and climate context of an active sequence of volcanic eruptions from this volcanic system. We found a rapid increase in volcanic activity following an abrupt warming event that led to the disappearance of marine terminating glaciers, supporting the idea that volcanism can respond quickly to regional changes in climate. We also found evidence for rapid fluctuations in sea surface variability, suggesting that climate can both force and respond to volcanism on a range of time scales.
S.K. Praetorius, A.C. Mix, B. Jensen, D. Froese, G. Milne, M.D. Wolhowe, J.A. Addison, F.G. Prahl (2016). Interaction between volcanism, isostatic rebound, and climate in Southeast Alaska during the last deglaciation. Earth & Planetary Science Letters, 452:79-89.
Changes in Ocean Circulation
Large-scale ocean circulation patterns play a fundamental role in redistributing both heat and carbon between the atmosphere and deep-sea. Major sources of deep-water formation occur in North Atlantic and Antarctic regions today, imparting large amounts of heat to the atmosphere. Understanding how the strength and dominant source regions of deep-water formation has changed through time is fundamental to our understanding of regional climate dynamics and the partitioning of heat and carbon dioxide between the atmosphere and ocean.
By reconstructing the flow speed of bottom waters in regions that are bathed by Nordic Seas overflows, we can estimate how the strength of the North Atlantic deep water has changed through time. In a study conducted at Woods Hole Oceanographic Institution, we found that the overturning circulation was relatively strong during both the glacial and interglacial period (although they extended to different depths), but was reduced during times of inferred meltwater addition to the North Atlantic, which were also associated with regional cooling. This supports the idea that changes in North Atlantic ocean circulation can impact regional climate on millennial time scales.
S.K. Praetorius, J.F. McManus, D.W. Oppo, W.B. Curry (2008) Episodic reduction in bottom-water currents since the last ice age. Nature Geoscience 1: 449-452.
Similar to deglacial changes occurring in the North Atlantic region, major deglacial meltwater pulses have been routed into the North Pacific, such as the Missoula Megafloods. I have been reconstructing changes in surface salinity coupled with proxies for water mass ventilation to understand how ocean circulation in the North Pacific changed in response to deglacial freshwater perturbations.
Praetorius, S.K., Condron, A., Mix, A.C., Walczak, M.H., McKay, J.L., Du, J. The role of Northeast Pacific meltwater events in deglacial climate change. Vol. 6, no. 9, eaay2915 (2020)