Approximately 6.6 million years ago in the Eastern Equatorial Pacific a large increase in biogenic mass accumulation rates (MAR’s) occurred. This increased level of biogenic mass accumulation persisted until about 4.4 Ma at which time levels returned to those similar to before the transition at 6.6 Ma. The exact nature of the change that facilitated this transition in biogenic MAR’s, however, was not understood. Here we present the results of a study which characterizes the diatom assemblage spanning the depositional transition at 6.6 Ma from sediments taken from ODP Hole 850B. A close inspection of lithology reveals a clear change in lithology from a diatom nannofossil ooze to a nannofossil diatom ooze at 6.6 Ma. This transition is immediately followed by the occurrence of laminated diatom ooze (LDO). Diatom absolute abundance data reveals three levels of productivity associated with pre-transitional, post-transitional, and LDO formational sediments. An increase in the absolute abundance of Thalassionema nitzschioides was the major contributor to the formation of post-transitional, and LDO sediments. The known ecological preferences of this species indicate an overall increase in nutrient availability followed by sporadic changes in nutrient availability. These changes in nutrient availability could be associated with the establishment, or increase in strength, of an upwelling cell and/or the increase in nutrients made available via upwelling due to a shoaling of the thermocline.

Approximately 6.6 million years ago in the Eastern Equatorial Pacific a large increase in biogenic mass accumulation rates (MAR’s) occurred. This increased level of biogenic mass accumulation persisted until about 4.4 Ma at which time levels returned to those similar to before the transition at 6.6 Ma. The exact nature of the change that facilitated this transition in biogenic MAR’s, however, was not understood. Here we present the results of a study which characterizes the diatom assemblage spanning the depositional transition at 6.6 Ma from sediments taken from ODP Hole 850B. A close inspection of lithology reveals a clear change in lithology from a diatom nannofossil ooze to a nannofossil diatom ooze at 6.6 Ma. This transition is immediately followed by the occurrence of laminated diatom ooze (LDO). Diatom absolute abundance data reveals three levels of productivity associated with pre-transitional, post-transitional, and LDO formational sediments. An increase in the absolute abundance of Thalassionema nitzschioides was the major contributor to the formation of post-transitional, and LDO sediments. The known ecological preferences of this species indicate an overall increase in nutrient availability followed by sporadic changes in nutrient availability. These changes in nutrient availability could be associated with the establishment, or increase in strength, of an upwelling cell and/or the increase in nutrients made available via upwelling due to a shoaling of the thermocline.

The ocean reanalysis Simple Ocean Data Assimilation (SODA) 2.2.4 is used to explore the changes in upwelling from normal conditions to either El Nino or La Nina conditions. Physical and thermodynamic variables from the reanalysis are used to explore the structure and behavior of El Nino Southern Oscillation (ENSO) events. The results of this analysis show that sea surface temperature (SST), entrainment velocity, wind stress, mixed layer depth, wind curl, and heat content anomalies are in general agreement with ENSO theory. Interestingly, the distribution of upwelling based on the entrainment velocity is very patchy, which led us to explore zonal and meridional sections of vertical velocity. We used three methods to compute changes in upwelling during ENSO events. The first method computes upwelling within the areas of SST anomalies during ENSO events. During El Nino events upwelling shows prominent decadal variability, while during La Nina the decadal variability is weaker. A new upwelling index is used for the second method, and upwelling is computed in the areas of strong upwelling anomalies. The variability of upwelling is higher in periods of reduced upwelling than in periods of strong upwelling. Despite the fact that the new index is computed independently, it agrees in the timing of the index used to define ENSO events for this research. The first and second methods show that the amplitude of SST anomalies and upwelling anomalies do not have a direct relationship, suggesting that upwelling does not explain all of the variance in SST. The last method used is to compute changes in upwelling in the Nino 1+2 region during ENSO events. In the east Pacific there is almost no correlation between upwelling and SST anomalies during ENSO, but this might be attributed to the fact that the Nino 1+2 region is a relatively small region compared to the Nino 3.4 region that is used to define ENSO events. In general, the time series of SST and upwelling anomalies agree well just in the cases when ENSO events are prominently in the eastern Pacific. A comparison between yearly fisheries data from Ecuador and Peru and monthly data of SST anomalies during ENSO years is presented showing that during El Nino events the fish catch decreases and during La Nina events the fish catch increases. We infer that the increase or decrease in fish catch is associated to changes in fish populations, and that these changes are mainly due to availability of nutrients and changes in temperature during ENSO events.

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