Past Global Carbon Cycle

Nature of Project(s)

Experimental (main) in combination with models (minor)

Essential Background:

EMSC3027/6027/8027 (Palaeoclimatology and Climate Change), EMSC8022 (Advanced Analytical Techniques), or EMSC 8024 (Foundations of Analytical Techniques and Data Science)


Ice core records show large and systematic variations in atmospheric CO2 concentrations on both orbital and millennial timescales. These records have invoked intensive research over the last decades, which has substantially expanded our knowledge about the mechanisms controlling past global carbon cycle. This knowledge is extremely valuable to enhance our confidence with future climate modelling in face of rising atmospheric CO2.

Despite the effort, scientists still cannot fully explain the reasons affecting past atmospheric CO2 fluctuations, warranting further research. The ocean is the largest carbon reservoir of the atmosphere-land biosphere-ocean system, and hence plays a critical role in affecting the atmospheric CO2 variability. At present, ocean carbonate chemistry records are very limited, creating a knowledge gap that limits our understanding of the global carbon cycle.

This group employs the state-of-the-art facilities at RSES to make precise geochemical measurements to reconstruct the past ocean chemistry conditions for both surface and deep oceans on various timescales. For the surface ocean, pCO2 reconstructions will allow us to evaluate past changes in CO2 absorption/outgassing intensities for key oceanic regions such as the Southern Ocean. For the deep ocean, nutrient and carbonate ion chemistry will provide critical clues about carbon storage changes associated with major water masses through time.

Possible Future Research Avenues:

  1. Application of boron isotopes to reconstruct pH and pCO2. Through intensive method development, we have (finally!) established a reliable method to measure boron isotopes of small biogenic carbonate samples (e.g., corals and forams) by using Neptune MC-ICP-MS and a brand-new prep FAST MC. This project aims to make the best use of the method to measure various samples from critical locations. The obtained boron isotope data will be considered along with other geochemical data (e.g., radiocarbon and nutrients) to gain insights into factors controlling past atmospheric CO2 changes on both rapid and long-time scales.
  2. Holocene climate-carbon coupling. Compared to the last deglacial period, the Holocene (~0 - 10,000 years ago) climate has remained relatively stable. However, there have been significant changes in ocean circulation and carbon cycle as registered by high-sedimentation archives. For example, ice core records show ~20 ppm increase in atmospheric pCO2 during the last ~8,000 years, but the mechanisms responsible for this change and their links to climate/ocean circulation remain elusive. To investigate Holocene climatic changes, high-sedimentation-rate cores are necessary. We have secured sediment cores with sediment rates up to 50 cm/kyr from key locations in the Atlantic Ocean with which we hope to provide new insights into processes controlling the Holocene climate and carbon cycle. The project aims to use various paleoceanographic tools to reconstruct mid- and deep-depth physical (temperature, salinity) and chemical (nutrient and carbonate ion concentrations) properties at various locations. Along with the use of intermediate complexity models (e.g., LOVECLIM, UVIC, etc), these new data will afford to identify the role of the Atlantic Ocean in controlling the regional and global climate during the Holocene.
  3. North Atlantic meltwater fingerprints. Ocean circulation is thought to have played an important role in the global climate by redistributing heat and affecting the global carbon cycle and hence atmospheric CO2 on Earth. Model studies suggest that melt water fluxes in the North Atlantic can impose significant impacts on the strength of Atlantic Meridional Overturning Circulation (AMOC). However, detailed meltwater history remains elusive. Meltwaters from glacials are characterized by negative oxygen isotope ratios, which offers a fingerprinting method to detect past meltwater inputs. This project aims to use paired planktonic foraminiferal Mg/Ca (a temperature proxy) and d18O (affected by temperature and seawater d18O) to calculate past seawater d18O. High-sedimentation cores (~30 cm/kyr) will be used to reconstruct detailed seawater d18O changes during the last deglacial period with particular emphasis on Heinrich Stadial 1 and Younger Drayas. The results will be interpreted in combination with an intermediate complexity model (e.g., LOVECLIM) to infer the influences of meltwater fluxes on past AMOC and global climate changes.
  4. Southern Ocean nutrient and atmospheric pCO2. It is well known that Southern Ocean processes play a critical role in affecting past atmospheric pCO2 changes. Surface nutrient levels are important to understand the biological pump efficiency in the past. However, despite extensive effort, it remains highly debated regarding the history of the Southern Ocean nutrient changes. One reason is that scientists are short of cores with enough carbonate materials to work on. This project aims to apply geochemical proxies (e.g., trace element and isotopes) to cores collected from Polar Antarctic Zone (~70oS) near the deep water formation regions. The results will provide an opportunity to gain new insights into mechanisms responsible for past atmospheric pCO2 changes.