Research

Our ability to predict future climate conditions relies on a keen understanding of the climate system and the sources of past climate variability. The brevity of the instrumental record and the time-scale of past climate changes compels us to utilize the geologic record to reconstruct Earth’s climate history. My research focuses on climate variability on timescales relevant to human societies, in particular interannual-decadal scale climate variability and its relationship to different background climate states. Extreme events on these timescales can have major societal impacts, and it remains unclear how strongly this variability is forced by boundary conditions. Our current research investigates the behavior of the El Niño Southern Oscillation (ENSO) through the Holocene and late Pleistocene, modes of tropical Atlantic variability during the Holocene, changes in tropical mean state (including surface and subsurface change), and how various proxy data record past conditions. I am also extensively involved with developing and testing a novel method for reconstructing variability using geochemical measurements of individual foraminifera.

The El Niño Southern Oscillation - ENSO

The largest source of inter-annual climate variability on Earth is the El Niño Southern Oscillation (ENSO). El Niño and La Niñ events are characterized by significant changes in tropical Pacific sea surface temperatures, with far-reaching effects felt across a wide portion of the globe as a result of the strong ocean-atmosphere teleconnections. The dramatic temperature and precipitation changes that occur during strong ENSO events can affect the health, safety, and food security of millions if not billions of people. Our understanding of the climatic factors that influence the severity and frequency of ENSO is incomplete, in part due to the relative brevity of the instrumental record. To better understand, characterize, and ultimately predict the response of ENSO to future climate conditions, we can look at how ENSO has responded to past changes in global climate. My research explores the mechanisms that affect ENSO variability and the relationship of ENSO to large-scale changes in the climate system which are fundamental to understanding tropical Pacific variability through the Holocene and on longer glacial-to-interglacial time-scales through the late Pleistocene.

Major Topics of Research

• Tropical Pacific variability on human time scales
  The last millennium has seen large-scale climate shifts including the Medieval Climate Anomaly and Little Ice Age. We identified a mid-Millennial shift in tropical Pacific dynamics from a low-variability, cool state to a high-variability, warming state using stable isotopes from over 1400 individual foraminifera and Mg/Ca to reconstruct SST. (Rustic et al, 2015)
• The Relationship of the mean state of the tropical Pacific Ocean to global climate states
• ENSO response to climatic boundary conditions on millennial and glacial-to-interglacial timescales (Rustic et al. 2020)
• Individual foraminifera variability and methodology (Rustic et al. 2021)
• Modes of Atlantic variability and the impact on African hydroclimate over the Holocene

Paleoclimate Tools and Methods

Studying the large-scale, long-term history of ENSO is done by looking at very smallest things. The geochemical signature of single-celled planktonic organisms called foraminifera, found in deep-sea sediments, can be used to reconstruct past oceanic conditions.

Aggregate analysis results in a single mean value for all of the foraminifera from an interval. Various population-level statistics can be obtained from individual analysis and compared across intervals to observe change in multiple dimensions. 

 These magnificent little geochemists have shells, or tests, made of calcium carbonate (CaCO₃). The oxygen isotope ratio (δ¹⁸O) and the ratio of Mg to Ca in the test is related to the calcification temperature and water properties (e.g., salinity and δ¹⁸O_sw). As each individual foraminifer has a life-span of roughly one month, careful analysis of the δ¹⁸O or Mg/Ca from individual shells provides multiple monthly "snapshots" of ocean conditions. With enough of these "snapshots", it is possible to infer not only the mean conditions, but also the variability of the ocean for the time that the foraminifera were alive. This climate information can then be compared to other time intervals to infer changes in ocean's mean state and its variability through time.

 High-resolution image of an individual T. sacculifer before (left) and after (right) laser ablation. Target size is 50 µm. A version of this is found in Rustic, et al. 2020 "Modulation of late Pleistocene ENSO strength by the tropical Pacific thermocline", Nature Communications 11, 5377. 

In addition to δ¹⁸O and Mg/Ca, I use other geochemical tools to reconstruct past ocean conditions. Mg replaces Ca as a function of calcification temperature, and thus the Mg/Ca ratios relate to the temperature during the foraminifera's lifetime. We can measure Mg/Ca ratios on aggregate samples of multiple crushed and cleaned foraminifera to provide us an estimate of the average temperature from the time interval we are studying. In order to measure the variability of ocean temperatures, we obtain the Mg/Ca ratios from individual foraminifera using laser ablation and inductively coupled mass spectroscopy (LA-ICPMS). This technique allows us to obtain detailed, chamber-by-chamber measurements of various trace metals (Al, Mg, Ca, Mn, Zn, Sr). Our investigation has shown that these chamber-to-chamber differences can be significant, and that the best practice for using T. sacculifer is analysis of Mg/Ca from multiple chambers to best match Mg/Ca resulting from analysis of the whole individual by solution chemistry means (Rustic et al. 2021 Relationship between individual chamber and whole shell Mg/ Ca ratios in Trilobatus sacculifer and implications for individual foraminifera palaeoceanographic reconstructions, Scientific Reports, 11:463)

 Example of TRaCES output showing the raw trace metal traces, integration area of the traces, buffer zones, and running Mg/Ca values. 

I have written software (TrACES - Trace metal Automated Computation and Evaluation Script) to automatically calculate Mg/Ca ratios from the large amounts of raw data generated by LA-ICPMS that detects characteristic changes in the data stream, applies drift correction, detects and flags possible contaminants, and summarizes the data by chamber and by individual. Working with the top-notch laboratory staff at LDEO, I have developed methods for measuring the Mg/Ca ratios in individual foraminifera using solution chemistry techniques and modified chemical cleaning procedures. We have begun using Optical Emission Spectroscopy (OES) ICP-MS to measure individual foraminifera, which will allow for the use of the individual foraminifera method during time periods where oxygen isotopes may be adversely affected by changes in seawater δ¹⁸O from ice volume effects, large scale salinity changes, or other factors.