One of my primary interests has been paleoceanography of the California margin. Over the past few years my group has been studying the history of changing marine productivity and carbon cycling along the California margin. There has been considerable interest directed towards the reconstruction of intermediate water ventilation history of the North Pacific, particularly on time scales associated with the abrupt climate changes observed in the Greenland Ice cap during the last glacial cycle (Dansgaard-Oeschger Events). The distribution of anoxic (laminated) and bioturbated sediments appears to co-vary with abrupt, large scale climate oscillations in the northern high latitudes. My group has been working to understand what causes the changes in sedimentation along the margin in connection with the climate variations. I was one of the proponents who wrote the ODP proposal that lead to deep coring along the California margin (LEG 167). These long cores provide a record of ocean and climate variability extending back to the middle Pleistocene. We have also been conducting a variety of sampling and analytical programs applied to the sediment record from the California margin in an attempt to unravel the relationship between climate and oceanographic variability along the margin.

Photo of Lowell and Peter preparing to deploy a piston core during a Site Survey Cruise for ODP Leg 167 on the California margin

Photo of multicore being deployed during a cruise in November 2001

One intriguing example of our findings has been the observation that the zone of anoxic and laminated sedimentation along the margin has expanded and contracted in size from time to time. Previously we had thought that these alternating sedimentation patterns occurred independently of climate changes and therefore were unique from those in the Pleistocene. However, we now know that these changes in sedimentation reflect changes in carbon oxidation in the basin due to varying marine productivity. These changes in productivity can now be attributed to changes in climate that affected upwelling along the California margin. Furthermore, we have learned that these recent changes were linked to Pacific-wide climate changes. The work we have done to understand the processes that bring about changes in laminated and bioturbated sedimentation in these basins gives us a better understanding of what controlled the patterns of sedimentation change during the Pleistocene, which have been correlated with the millennia scale oscillations in climate (Behl and Kennett, 1996) referred to as the Dansgaard/Oeschger cycles.

We now see the changes in Pleistocene sedimentation patterns in terms of changing atmospheric conditions over the north Pacific that influenced northerly wind and ekman upwelling along the margin. The periods of laminated sedimentation that correlate with warm-stadial conditions over Greenland reflect periods when the North Pacific high pressure system was strong and situated over the northeastern Pacific (as it is today during Spring and early Summer). This brought about stronger northerly winds during the Spring and early Summer and higher productivity in the basins along the California margin. Higher productivity promoted dysoxic conditions and laminated sedimentation. Colder stadial conditions over Greenland were associated with reduced high pressure in the north Pacific and lower productivity. Consequently, the basins were not depleted of oxygen because the flux of organic carbon was reduced. This promoted the incursion of bioturbating organisms and the deposition of bioturbated sediments in the basins.

We further find that the changes in Pleistocene sedimentation along the California margin can be related to the changes in atmospheric convection of the western tropical Pacific as outlined above. This is because the strength of the high pressure system in the north Pacific is ultimately related to the atmospheric convection in the western tropical Pacific. When this convection is reduced as it is during El Ninos (and as it apparently was during stadials) this results in weaker Hadley cell and reduced high pressure over the north Pacific. This link as been referred to as the "atmospheric bridge".

Below is a brief outline of the ongoing work we are doing on modern sediments that are helping us constrain the factors controlling the cycling of carbon within the basins along the California margin.

Modern varved sediments from the center of Santa Barbara Basin

The pattern of expanding laminated sediments seen in the plot above came to an abrupt end in 1976 (based on Pb210 dating). In the plot below you can see the tops of cores from basins all along the Califonia margin are bioturbated. The onset of bioturbation coincides with a Pacific-wide shift in climate and ocean circulation in the late 1970s, which resulted in a shift to a warmer and less biologically productive waters in California Current System.

In the Figure above the transition from laminated to bioturbated sedimentation occurs at essentially the same time, around the late 1970s in each of the basins. The transition occurs at different depths because of different sedimentation rates in each basin.

Using a variety of geochemical, biological and sedimentological approaches we have begun to study evaluate what caused the shift in sedimentation observed along the northeastern Pacific margin. For example the stable isotopic composition benthic foraminifera that inhabit different depths within the sediment allows us to reconstruct the pore water isotopic gradient, the magnitude of which is related to the amount of carbon oxidized. We find that the gradient increased significantly in conjunction with the spread of laminated sedimentation in these basins and then was reduced again in the late 1970s when the basins once again started to be bioturbated. The gradient decreased in the late 1970s at the same time that bioturbated sediment returned to the marine basins (see plots below). We also find that the accumulation of marine organic carbon and biogenic carbon changed in concert with the pore water gradient. Together these records are telling us that the changes in sedimenation are being caused by variable marine productivity and changing carbon oxidation within the basin. As a result oxygen utilization is be modulated by climate driven productivity changes.

Doug Hammond (on the right) during a coring cruise on the California margin showing graduate students Yi, Dave and Rebecca how to conduct the 'age-ole' taste test on a sediment sample.

We also find that other proxies, including the d15N and d13C of marine organic matter extracted from the sediments as well as the d13C of planktonic foraminifera also varied in concert with the changes in pore water gradient.

The figure at the top shows the live distribution of two species of benthic foraminifera in Santa Barbara, Santa Monica and basins in the Gulf of Mexico. Note that the two species inhabit different depths in the sediment. The d13C of the benthic foraminifera turns out to be a good proxy for the isotopic composition of pore water TCO2. We use the foram d13C to reconstruct changes in the pore water gradient over time (lower panel). Note how the pore water gradient increased from about 1700 with a dramatic increase in the 1950s and 1960s and then began to decrease again around the late 1970s. The increased pore water gradient coincided with the onset of laminated sedimentation at this site about 300 years ago.These data together with organic carbon accumulation rate changes point to changes in marine productivity as the cause of the expanding laminated sediments. Higher rates of carbon oxidation lead to further depletion of bottom water oxygen levels, which excluded bioturbating organisms.

We have begun to model the changes in pore water gradient in order to estimate how much carbon oxidation would be required to affect the pore water gradient. We also use this carbon oxidation model to estimate how much oxygen is consumed in the process. The preliminary results suggest that the changes in pore water isotopic gradient recorded in Santa Monica basin amount to a significant change in carbon oxidation on the sea floor and could easily account for a drop in bottom water oxygen in this silled basin required to exclude bioturbating organisms.

Preliminary pore water model showing how changes in the pore water isotopic gradient can be used to estimate the amount of carbon oxidation.

We hypothesize that changes in productivity affect the amount of carbon rain to the sea floor and this affects carbon oxidation rates, thereby affecting bottom water oxygen levels. If our hypothesis is confirmed it raises the distinct possibility that the role of primary productivity and carbon oxidation has been underestimated in previous studies of Pleistocene intermediate water ventilation history in the Northeastern Pacific which have called upon changes in dissolved oxygen to explain the periodic occurrence of laminated and bioturbated sedimentation.

I am particularly excited to be working on this problem as new climatological evidence from NASA suggests another flip in the PDO index may be occurring now. This climate shift may herald the onset of another cold phase in the California Current system and return of higher productivity with increased carbon rain. If our hypothesis is correct this will result in higher rates of carbon oxidation at the sea floor and lower bottom water oxygen levels in the basins which promotes the preservation of laminated sediments. We have just completed another research cruise in November/December of this past year funded by the National Science foundation during which we sampled a suite of dysoxic basins in the northeastern Pacific. This included Santa Barbara Basin and several basins within the Gulf of California.. Each of these silled basins contains laminated sediments but each is characterized by varying levels of dissolved oxygen and each receives differing amounts of carbon rain in response to marine productivity. These records will allow us to extend our proxy to a range of different environments and explore how wide spread the changes we saw in Santa Monica Basin were during the past several thousand years.