Hydrothermal cooling of midocean ridge axes: Do measured and modeled heat fluxes agree?

Accurate estimates of near-axis hydrothermal cooling are keys to understanding many geological, chemical, and biological processes at midocean ridges. At present, however, field observations (H, heat flux/vent field) are few and imprecise, and model results (Hm, heat flux/km of axis) vary widely depending on assumptions about crustal formation, crustal composition, and the cooling efficiency and spatial and temporal extent of hydrothermal circulation. In this paper I compare all published vent-field-scale measurements of H to a range of published Hm for slow (≤ 40 mm/yr) and fast (> 40 mm/yr) ridges to assess the present state of knowledge about the magnitude of near-axis hydrothermal cooling. H has been evaluated 28 times, either by point measurements using inventories of individual sources (362 ± 552 MW, n = 12 measurements), or by integrated measurements using water column observations (876 ± 1011 MW, n = 16). Comparable estimates of mean vent field “expected H” at any spreading rate can be derived by merging Hm from various models with the predicted global inventory of active vent sites. These estimates range from ∼ 1–1.5 GW/field and are approximately constant for all spreading rates. This range agrees with the best-constrained results of H from slow-ridge vent fields (1669 ± 1354 MW) but is considerably higher than those from fast ridges (245 ± 170 MW). Inferences about the spatial and temporal variability of H can be gained by defining the parameter L = H / Hm, equivalent to the axis length needed to continuously power a hydrothermal field. The resultant large L values (13–333 km for slow ridges, 1–32 km for fast) demand punctuated cooling, where for short periods H > Hm. The percentage of time that a given vent field might be active can be speculatively estimated by dividing L by the length of axis the field cools. Examples of this percentage range from a low of < ∼ 5% for a slow-ridge field to > ∼ 50% for some fast-ridge fields.