A latitudinal productivity band in the central North Atlantic over the last 270 kyr: An alkenone perspective

Productivity changes in the central North Atlantic Ocean have been traced by means of the total C-37 alkenone contents along two sediment cores located at 43 degreesN and 37 degreesN. Both alkenone signals revealed the occurrence of discrete productivity events every 23 kyr. Spectral analyses highlight the presence of a dominant 23-kyr periodicity in the alkenone signal, which is highly coherent to the precession index. However, a close comparison revealed small but relevant differences in the timing of several of the productivity events recorded at both locations. These asynchronies suggest that the alkenone maxima do not necessarily reflect a general increase of productivity over the North Atlantic. We propose that the events are related to a latitudinal band of productivity that moves northward and southward over time. Satellite-derived productivity estimates show that the present location of this band is 45 degrees -55 degreesN. To illustrate this hypothesis we have constructed a conceptual model that reconstructs the temporal changes of productivity at one given location by assuming a productivity band that evolutes latitudinally over time. The model is able to reconstruct the main features of the alkenone records, namely, (1) the occurrence of discrete and abrupt productivity events, (2) the asynchrony at different latitudes, and (3) the bimodal pattern of many of the productivity peaks.