Coral-based reconstruction of sub- to interdecadal climate variability in the Western Atlantic Warm Pool and the link to past hurricane activity
Final Report Abstract
Surface ocean temperature variability in the Atlantic affects the climate system across much of the Northern Hemisphere. During the 20th century sea surface temperature (SST) displays an upward trend that has been linked to anthropogenic global warming. Despite the wide-ranging effects of the warming trend on different aspects of the climate system, little is known about the seasonal variability of SST before the short record of existing temperature measurements. Proxy data that can provide insight into climate variability before that time are often limited by their temporal resolution and a lack of calibrations with in situ temperature measurements. Thus, it is difficult to draw conclusions about past seasonal- to decadal-scale climate variability and distinguish the contributions of man-made warming from natural low-frequency climate variability. In this project, we have used long-lived corals from the Cayman Islands, a location in the central Caribbean Sea, to develop records of past surface ocean conditions from the geochemistry of the corals carbonate skeletons. These corals form annual growth layers similar to tree rings and allow us to reconstruct seasonal-scale environmental changes, interannual to decadal-scale climate variability, as well as long-term trends. We have analyzed three coral cores that were drilled underwater from living Diploria strigosa colonies. The corals grow in two different reef environments, the forereef and the lagoon/backreef, allowing us to study reef-scale temperature variability that is commonly not resolved by satellite SST data. We have used a combination of coral oxygen isotopes (δ18O) and Sr/Ca ratios to study interannual to decadal-scale changes in water temperature. The oldest core dates back to the late 19th century, representing the longest seasonal-scale proxy-record of surface ocean conditions from an open-ocean site in the central Caribbean. Our results indicate that regional long-term warming was much stronger than displayed by coarse-resolution gridded SST products, that also lack data in the early part of the 20th century. Combined geochemical coral proxies suggest not only long-term changes in surface temperature, but also in the hydrological cycle. By comparing two corals from different, but neighboring, reef environments we have assessed the impact of warming events on reef-scale temperature variability. For the first time, we were able to calibrate our geochemical proxy records with high-resolution in situ measured temperatures over several years, an important step towards improving coral paleotemperature proxies. We show that susceptibility of corals to temperature stress not only varies significantly on local and regional scales, but also that reef-scale SST variability can be much larger than suggested by satellite-based SST alone. Depending on their position in the reef, corals of the same species may respond differently to short-term temperature stress events, potentially also influencing coral growth and geochemistry. We could also confirm the influence of external climate signals (e.g. El Niño-related warming events) on past regional SST variability recorded in coral proxies. Surprisingly, data from the shallow water lagoon indicated a possible influence of long-term tidal cycles on sub- to interdecadal climate variability recorded in coral Sr/Ca, an effect which has been hypothesized before, but could not be proven to date. In addition to the initially proposed coral proxies, we had the opportunity to analyze coral nitrogen isotopes (δ15N), a newly developed proxy for the reconstruction of past nitrogen cycling. Through a collaboration with Dr. Yamazaki (University of Tokyo, Japan), who developed this novel proxy, our longest coral core yielded the first annual resolution record of coral δ15N from the tropical Atlantic. This has allowed us to reconstruct past nitrogen fixation rates of the surface ocean, which play an important role in oligotrophic oceans controlling primary productivity, adding a completely new aspect to our project. Until now, few direct measurements of nitrogen fixation rates exist, decadal to centennial scale variability of nitrogen fixation rates is still unknown, and the controlling factors in the North Atlantic ocean are debated controversially. We found that nitrogen fixation rates in the Caribbean Sea decreased during the past century, while SSTs increased. In addition, we could for the first time document that nitrogen fixation rates in the North Atlantic vary on decadal- to multidecadal scales in concert with water temperature and hurricane activity. Coral δ15N data suggest that ocean circulation may drive the century-scale variation of nitrogen fixation in the Caribbean Sea. These are exceptional results that will help to better understand tropical surface ocean nutrient dynamics. In summary, our findings provide new input data for validating results from models studying present and future climate change, in particular rising SSTs, which are an important variable in ocean and atmospheric processes.
Publications
- (2013) Calibration of coral Sr/Ca and δ18O using high-resolution in situ temperature data: A case study from the Cayman Islands, Caribbean Sea. In: 3. International Sclerochronology Conference, 18.-22.05.2013, Caernarfon, England
von Reumont, J., Hetzinger, S., Garbe-Schönberg, D., Manfrino, C., Grove, C. A. und Dullo, W. C.
- (2014) Assessing the influence of sea surface temperature on coral growth and Sr/Ca: A replication study from the Cayman Islands. In: AGU Ocean Sciences Meeting, 23.-28.2.2014, Honolulu, USA
von Reumont, J., Hetzinger, S., Garbe-Schönberg, C. D. und Manfrino, C.
- (2014) δ15N in Caribbean coral skeletons: Decadal variability in North Atlantic nitrogen fixation. In: AGU Ocean Sciences Meeting, 23.-28.2.2014, Honolulu, USA
Yamazaki, A., Hetzinger, S., von Reumont, J., Manfrino, C., Tsunogai, U. und Watanabe, T.
- (2015) Tidal influence on lagoonal temperature in Little Cayman, Central Caribbean, using high-resolution in-situ temperature measurements. In: ASLO Aquatic Sciences Meeting 2015, 22.-27.02.2015, Granada, Spain
Degregori, S., Hetzinger, S., von Reumont, J., Manfrino, C., Jacoby, C. und Luis, K.
- 2016, Impact of warming events on reef-scale temperature variability as captured in two Little Cayman coral Sr/Ca records, Geochem. Geophys. Geosyst., 17
von Reumont, J., Hetzinger, S., Garbe-Schönberg, D., Manfrino, C., Dullo, W.-Chr.
(See online at https://doi.org/10.1002/2015GC006194) - Decline in 20th century Caribbean nitrogen fixation archived by annual resolution coral record. GeoBremen 2017, 24.-29.09.2017, Bremen, Germany
Yamazaki, A., Hetzinger, S., von Reumont, J., Manfrino, C., Tsunogai, U., Sano, Y., Watanabe, T.
