Paleoclimate Variability Working Group Summary

The primary focus of OCTET is to improve our understanding of biogeochemical processes related to the carbon system that must be known to estimate future global environmental changes. The prime goals of OCTET encompass a longer time frame than did the goals of JGOFS. Paleoceanography (the study of the behavior of the ocean in the past) can contribute to OCTET by extending our understanding of natural variability of the ocean to the period before the onset of modern instrumental observations. To do this, the tools of paleoceanography - which in recent times are dominantly geochemical "proxy" tracers - must be properly calibrated and evaluated. The observations made by OCTET will be crucial in establishing and improving these tracers when they are made in the context of the assessing appropriate paleoceanographic proxies.  

How paleoceanography can contribute to OCTET

The scientific community is attempting to predict the consequences of global anthropogenic perturbations for decades to centuries into the future. Most observations of the ocean extend only 50-100 years into the past, yet even for that period many observations are sparse and incomplete. It is risky to predict the evolution of even an unperturbed system for periods longer than the interval of observations. Many of the global changes anticipated for the future have occurred in the past; it behooves us to take advantage of these past natural experiments to improve our understanding of biogeochemical processes. If glacial/interglacial CO₂ cycles cannot be explained, how can we claim to have predictive skill for the carbon system?

We have already learned much about the carbon system from paleoclimatic records, particularly with regard to the processes that govern natural CO₂ variability. Our geologic and glaciologic records show that the carbon system can undergo large changes in response to relatively minor shifts in incoming radiation (Milankovich Effect). These data indicate that the sensitivity of the carbon system may be large, and indicate the presence of large positive feedbacks to amplify the modest and gradual energy forcings into major climate shifts. Evidence suggests that nutrient uptake efficiency in high latitude, high nutrient regimes has changed, and that there have been large changes in export flux from the surface ocean. Large changes in the physical environment of the upper ocean have been documented (e.g., temperature, salinity, mixed layer depth, thermocline depth); these changes have biogeochemical consequences that are documented in the sedimentary record. Scientific understanding that the earth is not a fixed unchanging environment is based on evidence from the past derived from geological studies; we might be more complacent about the prospects of future change without this geological perspective.

Concretely, paleoceanography can contribute the following to OCTET goals:

• The study of changes that have occurred in the past can contribute to the understanding of long term natural variability (against which future changes must be judged as natural or due to human perturbations)
  
  
• Instrumental time series for the past few decades can be extended into the past, allowing for (a) better judgment of periodic and aperiodic behavior, (b) identification of the long-term mean, and (c) understanding natural extrema.
  
  
• Most measurements of the ocean have been made after anthropogenic perturbation began; paleoceanographic tools can assess the state of the ocean before it was perturbed.
  
  
• It is convenient to assume that the ocean was in a steady state before human influences, but we cannot know this assumption is true without studies that evaluate the system before the perturbation began.
  
  
• In cases where the forcing factors are understood (e.g., orbital influences on insolation; changes in greenhouse gas concentrations), paleoceanographic studies can assess the sensitivity of climate to forced changes and evaluate the positive and negative feedbacks that occur in response to the initial forcing.
  
  
• Often the period of modern observations began after the injection of valuable transient tracers such as radiocarbon and tritium. Paleoclimatic studies can help us reconstruct the evolution of transient tracers through studies of tree rings. Examples: the evolution of tritium in tropical oceans may be recoverable from the tritium content of mangroves; corals contain records of some transient radioisotopes [⁹⁰Sr] and trace elements [Pb]; studies of ¹⁴C in surface and deep-sea corals can document the early evolution of the bomb radiocarbon transient; and studies of C in sclerosponges can document the oceanic history of the "Suess Effect", ¹³C reduction by fossil fuel burning.
  

Several points about time scales should be emphasized in this context:

1. OCTET may not want to consider all processes that operate on glacial/interglacial time scales (and longer).
   
   
2. Paleoceanographic measurements can cover both millennial and shorter time scales. Examples: (a) laminated sediments with annual resolution [e.g., diagenetic metals (Mo) in Santa Barbara Basin appear to provide a record of ENSO in recent sediments, and stable isotopes document centennial climate change 25-65 ka], (b) high deposition sediment records with centennial resolution (e.g., the "Little Ice Age" and "Medieval Warm Periods" have been observed in Bermuda Rise cores), and (c) scleractinian corals, deep sea solitary corals, and mollusk records that provide records of bomb ¹⁴C and other chemical changes.
   
   
3. Processes that are important to glacial/interglacial CO₂ change can act in many cases on any time scale, even if their natural forcings occur on longer time scales. Example: changes in high latitude nutrient uptake.
   
   
4. We must be cautious about pigeon-holing the time scale of some processes. For example, an abrupt change in nitrogen fixation may cause rapid short-term changes in carbon flux before settling into the steady-state millennial response time of the oceanic nitrate inventory.

How OCTET can contribute to paleoceanographic work

At the same time that paleoceanographic work can contribute to OCTET goals, OCTET studies can contribute to the development and refinement of paleoceanographic tools. OCTET will improve our understanding of the biogeochemical character of the modern ocean, which will provide a context within which proxy tools can be compared to modern conditions. OCTET refinement of our understanding of geochemical processes will contribute to our ability to model past environmental changes, and reduce the uncertainties of paleoceanographic proxies. The best paleoceanography is done by iterative forays between proxy development in the modern ocean, investigation of paleo archives, and consideration of the implications for modern biogeochemistry.

OCTET can contribute the following to paleoceanography:

• Ground-truthing of recorders, proxy testing and development: OCTET can contribute to proxy testing in the context of process studies. For example consider the JGOFS studies of particulate and diagenetic barium fluxes, and process studies of N isotopes and nitrate uptake. In some cases we have major discrepancies between proxies. Proxies can be difficult to measure, so individual investigators may find it necessary to specialize in a limited number of properties; but these measurements should be considered together with other proxies at the same times and places rather than being completely disjoint in time and space. OCTET coordination can ensure that full advantage is taken of the multi-proxy approach.
  
  
• Some geochemical tracers have an inadequate global database (e.g., d¹³C in deep and bottom waters, nitrogen and silicon isotopes in the upper ocean, Cd and Fe concentrations in the water column). Future proxy development may depend on the availability of archived samples. A major effort such as OCTET will send biogeochemists to sea in remote locations. It is imperative that we take advantage of this presence to improve our global database of geochemical tracers. As far as is practical, OCTET cruises should archive water, particle, and box-core/multi-core samples with pore water geochemical measurements for future measurements of tracers yet to be developed.
  
  
• Paleoceanographic studies rely on the accuracy of time scales provided by radioisotopes such as ¹⁴C and ²³⁰Th. Although our understanding of these tracers is good enough to provide reasonable accuracy, evolving demands for improved precision and accuracy of dating exceed our understanding of the behavior of these radioisotopes in the modern ocean. OCTET will use radioisotope tracers such as ¹⁴C and ²³⁰Th to help constrain rates of processes in the modern carbon system, but OCTET can also use these studies to improve our understanding of the geochemistry of these isotopes so as to provide for more accurate and precise time scales in future paleoceanographic efforts.
  

Paleooceanographic studies appropriate for OCTET

The following examples (with testable hypotheses) indicate some of the paleoceanographic studies appropriate for OCTET. The list is not exclusive; the justification for a given paleo/OCTET link is best done by individual scientists explaining the relevance of their work to OCTET goals.

1. The relationship between sea surface temperature, salinity, sea ice, and other physical boundary conditions with paleoceanographic indices
   Hypothesis: Stratification changes in the Southern Ocean were forced by changes in the wind-field and associated shifts in Ekman divergence.
   
   
2. The relationship between geochemical tracers (e.g., ¹⁴C) and the physical circulation of global and regional oceans, and their role in determining surface ocean processes.
   Hypothesis: Climate change is manifested by the frequency within which recognized states of the modern climate system are occupied. Example: the "bipolar seesaw", first recognized in paleoclimate records, may affect the carbon and nutrient chemistry of the ocean subsurface, with changes occurring on decadal time scales.
   
   
3. The relationship between nutrient concentrations and uptake, and ecological regime in high nutrient areas. Changes in export flux from the surface ocean, and chemical composition of that export.
   Hypotheses: Nutrient uptake in the Antarctic surface was higher during the last ice age and therefore drove glacial/interglacial atmospheric CO₂ change. Export fluxes were several fold lower in the glacial Antarctic and several-fold higher in the glacial Subantarctic.
   
   
4. The relationship between Redfield ratios and nitrogen fixation.
   Hypotheses: The oceanic nitrate reservoir was modulated by the size of the inorganic phosphate reservoir. Nitrogen fixation rates were higher in the glacial ocean, and denitrification rates were lower.
   
   
5. The relationship between margin environments (high resolution sedimentary records and their contribution to export to the deep ocean), anthropogenic impacts on the ecology and productivity of coastal waters, and the role of rivers and estuaries.
   Hypothesis: Large natural variations have occurred in export production and/or the intensity and depth of the O₂ minimum zone in the California Current system.
   
   
6. (The relationship between multi-annual and decadal ocean variability will affect biogeochemical cycles, and the nature of these modes of variability will likely change in the future.
   Hypothesis: The frequencies of ENSO/NAO/PDO events have changed on centennial time scales.

Conclusions

The paleoclimatic record shows that large changes in oceanic "regime" have occurred (glacial/interglacial changes, variations in fish populations off Southern California, etc.). Although paleoclimate proxies are always subject to improved quantitative understanding, there is no doubt about the qualitative accuracy of these observations of oceanic variability. We must develop a better understanding to accurately interpret these changes.

Although we foresee a symbiotic relationship between OCTET and paleoceanographic research, paleoceanographic work within OCTET should focus on the main OCTET goals. OCTET should focus on proxy testing and development through process-related measurements (with box coring at process stations linking ocean process studies to studies of the sedimentary record; coral calibration studies near OCTET time series stations, etc.)