A mid‐Holocene transition in the nitrogen dynamics of the western equatorial Pacific: Evidence of a deepening thermocline?

Sedimentary δ15N records from the oligotrophic western equatorial Pacific (WEP) off Mindanao show that late Holocene sedimentary δ15N is substantially lower than that of the early Holocene, following a gradual >3 ‰ decrease that occurred between 7 and 3 kyrs ago. Analyses of modern day nitrate isotope profiles from the same region indicate the sensitivity of the WEP N pools towards (1) the advection of 15N‐enriched nitrate from the Eastern Equatorial Pacific (EEP) by the North Equatorial Current (NEC) and the Mindanao Current in subsurface waters and, (2) at shallow depths, the input of new and 15N‐depleted nitrate through N2 fixation. We suggest that the Holocene decrease in sedimentary δ15N reflects a diminished relative input of 15N‐enriched nitrate to the surface biota, either through an increase of regional nitrogen fixation, a change in nitrate consumption along the advective path of nitrate supply, or a decrease in the vertical supply of 15N‐enriched nitrate from the NEC. The latter mechanism is consistent with a Holocene deepening of the WEP nitracline/thermocline.

[1]  J. Waniek,et al.  Nitrate isotope anomalies reflect N2 fixation in the Azores Front region (subtropical NE Atlantic) , 2009 .

[2]  E. Guilyardi,et al.  ENSO at 6ka and 21ka from ocean–atmosphere coupled model simulations , 2008 .

[3]  M. McIlvin,et al.  Isotopic analyses of nitrate and nitrite from reference mixtures and application to Eastern Tropical North Pacific waters , 2007 .

[4]  A. Mix,et al.  A multiproxy assessment of the western equatorial Pacific hydrography during the last 30 kyr , 2007 .

[5]  A. Bush.  Extratropical Influences on the El Niño–Southern Oscillation through the Late Quaternary , 2007 .

[6]  G. Haug,et al.  Influence of the intertropical convergence zone on the East Asian monsoon , 2007, Nature.

[7]  Y. Yamanaka,et al.  Nitrate-nitrogen isotopic patterns in surface waters of the western and central equatorial pacific , 2006 .

[8]  D. Sigman,et al.  Origin of the deep Bering Sea nitrate deficit: Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes , 2005 .

[9]  D. Sigman,et al.  Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin , 2005 .

[10]  Gerald H. Haug,et al.  Isotopic constraints on glacial/interglacial changes in the oceanic nitrogen budget , 2004 .

[11]  G. Haug,et al.  Decline of surface temperature and salinity in the western tropical Pacific Ocean in the Holocene epoch , 2004, Nature.

[12]  A. Ishida,et al.  Seasonal and interannual variations of the North Equatorial Current bifurcation in a high-resolution OGCM , 2004 .

[13]  R. Thunell,et al.  Glacial‐Holocene δ15N record from the Gulf of Tehuantepec, Mexico: Implications for denitrification in the eastern equatorial Pacific and changes in atmospheric N2O , 2004 .

[14]  C. McClain,et al.  A ribbon of dark water: phytoplankton blooms in the meanders of the Pacific North Equatorial Countercurrent , 2004 .

[15]  D. Sigman,et al.  Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. , 2002, Analytical chemistry.

[16]  Athanasios Koutavas,et al.  El Niño-Like Pattern in Ice Age Tropical Pacific Sea Surface Temperature , 2002, Science.

[17]  Gerald H. Haug,et al.  Southward Migration of the Intertropical Convergence Zone Through the Holocene , 2001, Science.

[18]  C. Barford,et al.  A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater. , 2001, Analytical chemistry.

[19]  M. Lewis,et al.  Geographical distribution of new production in the western/central equatorial Pacific during El Niño and non‐El Niño conditions , 2001 .

[20]  M. Altabet.  Nitrogen isotopic evidence for micronutrient control of fractional NO3− utilization in the equatorial Pacific , 2001 .

[21]  R. Ganeshram,et al.  Glacial-interglacial variability in denitrification in the World's Oceans: Causes and consequences , 2000 .

[22]  B. Qiu,et al.  Seasonal and interannual variability of the North Equatorial Current, the Mindanao Current, and the Kuroshio along the Pacific western boundary , 1996 .

[23]  Syaefudin,et al.  The water masses between Mindanao and New Guinea , 1996 .

[24]  John J. Cullen,et al.  New production in the warm waters of the tropical Pacific Ocean , 1994 .

[25]  Y. Masumoto,et al.  Response of the Western Tropical Pacific to the Asian Winter Monsoon: The Generation of the Mindanao Dome , 1991 .