Decadal to centennial fluctuations in the intensity of the eastern tropical North Pacific oxygen minimum zone during the last 1200 years

Abstract

Oxygen minimum zones (OMZs), located below highly productive marine regions, are sites of microbially mediated denitrification and biogeochemical cycling that have global significance. The intensity of OMZs fluctuates naturally; however, the degree of these fluctuations and a comprehensive understanding of the factors that drive these fluctuations on decadal to centennial time scales is lacking. Our high-resolution (near-annual) record of δ 15 N sed from laminated sediments at the Pescadero Slope in the Gulf of California (eastern tropical North Pacific) fluctuates between maximum values of 10.5‰ and minimum values of 8.0‰ over the past 1200 years. An analysis of the relationship between δ 15 NO 3 À and [O 2 ] in the water column suggests that the observed range of δ 15 N sed values is equivalent to an approximately 8 μM fluctuation in O 2 content and that these changes can occur in less than 25 years. Our findings show that the OMZ typically intensifies quickly and contracts gradually; the average rate of OMZ intensification (À0.24 μM O 2 /yr) is twice as fast as the rate of OMZ reoxygenation. Spectral analyses of the δ 15 N sed record and Br/Cl counts, with the latter are used as a proxy for organic carbon preservation, suggest that the Pacific Decadal Oscillation and the Suess (deVries) solar cycle (solar irradiance) may influence the intensity of the OMZ and carbon production/export during the late Holocene. Coherence between δ 15 N sed and weight percent organic carbon also suggests that similar mechanisms influence both OMZ fluctuations and variation in organic carbon production/export.

Local and Remote Forcing of Denitrification in the Northeast Pacific for the Last 2,000 Years

“…Comparison with the Pescadero slope δ 15 N sed record in the Gulf of California (Tems et al, 2016) supports subsurface tropical water influences on the surface waters of the Southern California Bight. The Pescadero δ 15 N sed record suggests that ETNP denitrification was associated with ITCZ migration on multidecadal to centennial timescales and supports the stronger tropical water transport into Southern California during the MCA.…”

“…Yet the out-of-phase pattern during 1670-1840 CE (1720-1840 CE in particular) cannot be explained by age model offsets given its duration of centuries. Coincident with NH cooling during the Maunder (1645-1715 CE) and Dalton (1790-1820 CE) solar minimum, the 1670-1840 CE interval stands out as a period of equatorward ITCZ migration (YOK-I δ 18 O; Kennett et al, 2012) where the weakened ETNP OMZ resulted in low Pescadero δ 15 N sed values (Tems et al, 2016; Figures 8c and 8d), while SBB δ 15 N sed values increased (Figure 8b). As Pescadero δ 15 N sed values are always higher than those in SBB, this out-of-phase pattern reduces the δ 15 N difference between the two sites.…”

“…The elemental, isotopic, and organic geochemical composition of OC in SBB indicates marine sources with significant contributions from terrestrial OC, notably in flood sediments. Background SBB sediment δ 13 C values of −21.75‰ support a marine source, as terrestrial contributions (primarily from the Santa Clara Tems et al (2016). Red star: ice core location from Mount Logan (Osterberg et al, 2014); red triangle: the YOK-I speleothem record from Belize (Kennett et al, 2012).…”

“…Core locations. (a) Yellow circle: Core SPR0901‐03KC (34°16.99′N, 120°2.408′W; 586‐m depth) from the Santa Barbara Basin (SBB); red circle: Pescadero Slope core location from Tems et al (). Red star: ice core location from Mount Logan (Osterberg et al, ); red triangle: the YOK‐I speleothem record from Belize (Kennett et al, ).…”

“…(a) Mount Logan North Pacific Index proxy record (purple line,Osterberg et al, 2014). Lower values indicate stronger winter Aleutian Low; (b) δ 15 N sed from Santa Barbara Basin core SPR0901-03KC with the thick purple line showing the 11-point running mean to highlight multidecadal variability; (c) δ 15 N sed of the Pescadero Basin(Tems et al, 2016) with the thick red line denoting 11-point running mean;(d) YOK-I speleothem δ 18 O as a proxy for Intertropical Convergence Zone (ITCZ) migration(Kennett et al, 2012).The 41point running mean is shown in the black solid line. (e) Cross-wavelet coherence between δ 18 O in (d) and Pescadero δ 15 N sed in (c).…”