Paleo-oceanographic and Long-Term Historic Evidence of Past Variability

Participants: D. Ainley, M. Eakin, D. Hedgecock, A. Hollowed, M. Mullin, G. Rau, J. Rice, P. Smith, and L. Welling

Two major challenges that face GLOBEC are detecting ecosystem response to global change, and disentangling the effects of anthropogenic forcing from those induced by natural variability in the climate system. Knowledge of the past is vital to the design and implementation of programs to meet these challenges. Two exceptional and complementary sets of historical information - the high-resolution paleosedimentary record and the CalCOFI data set - offer a compelling argument for a California Current/eastern boundary GLOBEC initiative.

Several sites in the California Current contain anaerobic "sediment memories" of ecological variability over unusually short time scales of decadal, yearly, and even seasonal periods. Two confirmed sites and one potential site exist in the California Current system, plus an additional site in the Gulf of California, Mexico (Fig. 8). Information from these anaerobic sites has been used, for example, to reconstruct the biomass of northern anchovy and sardine populations over the last 2000 years, based on the abundance of their scales in the sediments of the Santa Barbara Basin (Fig. 9; Soutar and Isaacs 1974; Baumgartner et al. 1992). A variety of ecologic and geochemical information can also be extracted from sedimentary plankton remains to characterize the history of variability in the physical and biological environment in which the fish were living.

The CalCOFI data set documents the California Current ecosystem over the last four decades and offers a unique and ongoing opportunity to quantify the dynamics of populations and communities over interannual and decadal time scales. Integration of the paleoecological series and the historical CalCOFI data will be a powerful tool in the development of GLOBEC modeling and field sampling programs. Use of the CalCOFI information to interpret the high-resolution paleo-oceanographic record also helps us not just to describe, but also to understand the history of the California Current over the past century and back through at least two millennia. Conversely, modeling of ecosystem responses, based on present-day data, can be validated against the changes documented from the CalCOFI and the paleoecological data.

Figure 8. Circles in the upper left box indicate the known and potential sites for reconstructing the histories of coastal pelagic fish populations from anaerobic depositional environments. These are the fjords on the Pacific coast of Vancouver Island (VI; still untested for fish scales), the Santa Barbara Basin (SBB), the Soledad Basin (SLB), and the Guaymas Slope (GS) in the Gulf of California. The four charts indicate the principal areas of concentration and spawning of the four dominant coastal pelagic species during summer, according to the sources used by Ware and McFarlane (1989) for this compilation.

Figure 9. 1650-year proxy time series of biomass estimates for Pacific sardine and northern anchovy off California, based on the relationship between scale-deposition rates and modern population estimates (Baumgartner et al. 1992).

KEY RESEARCH PROBLEMS AND APPROACHES

1. Determine the characteristic time scales and rates of change in the California Current ecosystem that are associated with global climate change. This requires extensive and detailed analysis of accurately dated, high-resolution sediment records.

2. Calibrate paleo-oceanographic records of population dynamics and environmental change against sediment-trap records to monitor present-day patterns in particle fluxes. This is necessary to determine the ways in which population change is transmitted to and preserved in the sediments. Sediment-trap studies offer a unique perspective on the ecology of fish and plankton species and lay the groundwork for reconstructing environmental settings. These studies will allow us to select appropriate plankton taxa as proxies for specific hydrographic conditions (e.g., Sautter and Thunell 1991) and will greatly amplify the value of fossil plankton (foraminifera, radiolaria, and diatoms) and fish remains as records of population shifts that reflect changing environmental conditions.

3. Use historical data bases and sample archives from CalCOFI to document changes in populations and communities and their relationships to global climate changes. The Scripps Institution of Oceanography, the Southwest Fisheries Science Center of NOAA/NMFS, and the California Department of Fish and Game have archives of fish and plankton samples as well as ancillary hydrographic and biological data from the late 1930s to the present, with continuous surveys of key areas since 1951.

4. Extrapolate the basin-specific paleoenvironmental records from the Southern California Bight to population and environmental changes over a wider geographic region of the CCS using contemporary sediment-trap studies. Sediment-trap data can test the generality of the Santa Barbara Basin fish-scale record with respect to the larger habitat of the fish and can relate scale-deposition rates to current biomass estimates of target species.

5. Explore promising sites for high-resolution paleo-oceanographic records outside the Southern California Bight along the west coast of North America (Fig. 8). The silled fjords in British Columbia have not yet been adequately searched for preserved fish scales. The existence of a fish-scale record in Soledad Basin off southern Baja California has been documented (Soutar and Isaacs 1974) but not well developed. Exploration and development of the information from these sites will yield a fuller description of spatial variability in conjunction with the long-term records.

6. Integrate sediment-trap-based flux studies into a long-term monitoring program to collect continuous seasonal, interannual, and decadal information about the ecosystem. This program would be part of the long-term GLOBEC monitoring study outlined in section 3.7 of this report. Such monitoring of selected sites and variables should be continued at least 20 years after the intensive studies of GLOBEC are concluded, to make sure that contemporaneous ecosystem change is detected.

7. Use paleo-oceanographic and historical data sets to constrain and test oceanographic and population models. Past changes in fish abundances from the sedimentary record can be used to constrain contemporary models of climatic effects on fish population dynamics. The quality of the information now emerging will be improved by finer sampling intervals in the long record. We envision a detailed study (e.g., two-year sample intervals) comparing contemporary twentieth-century warming with the medieval warming event that occurred between A.D. 900 and 1300.

TOOLS AND METHODOLOGIES

Documentation and Interpretation of the High-Resolution Sedimentary Record

Analysis of Data Archives from Historical Collections

Finally, the relationship between scale-deposition rates from the Santa Barbara Basin sediments and the biomass of pelagic fish populations is still not well determined. There is only a brief overlap (eight data points using 5-year averages in Soutar and Isaacs 1974) between the existing sedimentary data and the fish population estimates (1932-69). We now have better population estimates and evidence of significant population changes for the period since 1970. Therefore, we can significantly improve calibration of scale-deposition rates by updating the sedimentary record and its relationship to the improved biomass estimates.

Calibration of the Sedimentary Record Through Sediment Trap Studies

These studies would be enhanced if done in conjunction with the intensive field studies of the GLOBEC program. If the intensive field studies were augmented with sediment-trap information, we could observe how planktonic responses to ocean physics and associated biological productivity are preserved in the sedimentary record . We also could determine if fossil plankton taxa could be used as proxies for other plankton (especially fish larvae) not preserved in the sedimentary record.