PHYSICOCHEMICAL CONTROLS ON FORAMINIFERAL SIZE


605


TABLE 3. The order (i.e., step) of environmental variables added to the model in order to best predict the distribution of (log10-transformed) volume–to–surface area ratio from Pacific waters with ≥3 ml/liter and <3 ml/liter of dissolved oxygen (top to bottom). AIC and BIC values are reported as the difference (∆) between the model in question and the model with the lowest reported value. Our model-selection process considers both AIC and BIC values, and if two models yielded similar values we selected the simpler model (i.e., the model with the fewer environmental predictor variables).


Predictor


Pacific ≥3 ml/liter Temperature Oxygen


POC flux


Calcite saturation state Best model


Pacific <3 ml/liter Temperature Oxygen


POC flux


Calcite saturation state Best model


Step


Null model 1 2 3 4


Null model 1 2 3 4


df 1


2 3 4 5


2 3 4 5


both Pacific and Atlantic waters with more than 3 ml/liter [O2], variations in temperature elicit the greatest change in test morphology due to their effect on metabolic rates. The general trend of the contour lines in


Deviance 102.3


101.3 101.2 101.0 101.0


104.3 102.8 100.3 100.2 100.2


∆AIC 15.7


0.7 0.1 0.0 1.9


log10 volume–to–surface area ~ temperature 1


58.6 37.5 0.0 1.8 3.8


log10 volume–to–surface area ~ temperature + oxygen


Figure 6 supports our findings from empirical test size and volume–to–surface area ratio data from North American continental margin. For purposes of illustration, we have assumed that other variables included in the model are constants. In nature, however, parameters such as organism metabolic rate and cytoplasmic streaming velocity vary among species and environments. Because cell morphology can be maintained under changing environmental conditions by varying these parameters, contour lines are not hard constraints on test volume–to–surface area but instead can be interpreted as lines of approximately equal selective pressure on it. We overlay our empirical data on Figure 6 by plotting the mean temperature and dissolved oxygen con- centration of foraminifera that have test volume–to–surface area ratios <0.05mm (square), 0.05–0.2mm (triangle), and >0.2mm (circle); benthic foraminiferal test volume– to–surface area ratios in the North American data set span 0.012mm to 0.32mm. The mean temperature and dissolved oxygen values for these three groupings of test volume– to–surface area ratio are consistent with the selective pressures implied by equation (2) and illustrated in Figure 6.


trations exert a significant influence over both the size and volume–to–surface area ratio of benthic foraminifera from the North American continental margin. Similar metabolic controls over these two different measures of morphol- ogy suggest that most of the response of benthic foraminiferal test volume–to–surface area ratios to environmental pressures is through changes in test volume rather than changes in volume–to–surface area ratio at a constant size. In other words, the primary mechanism by which foraminifera vary in surface area-to-volume ratio is by differences in size, rather than differences in the ratios of lengths of the primary axes. Thus, benthic foraminifera appear to use the same physiolo- gical mechanism for changes in test volume and volume–to–surface area ratio.


Temperature and dissolved oxygen concen-


Implications for the Benthic Foraminiferal Fossil Record


The metabolic controls that shape patterns in


the spatial distribution of modern benthic foraminiferal test size and volume–to–surface area ratio around the North American continental margin may shed new light on the causes of both short- and long-term patterns of morphological evolution in the foraminiferal fossil record. Specifically, the results presented herein show that temperature is the most important control in the physical environment


∆BIC 9.4


0.0 4.9


10.3 17.6


47.8 32.2 0.0 7.2


14.6


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