The Nitrogen Isotopic Record From the Peru-Chile Suboxic Zone; Distinguishing Internal

1. The Nitrogen Isotopic Record From the Peru-Chile Suboxic Zone; Distinguishing Internal and External Signals Across the Last Deglaciation 1SMAST, University of Massachusetts Dartmouth 2Geological Sciences, Brown University 3Institute of Marine and Coastal Sciences, Rutgers University M. A. Altabet1, S.C. Bova2, T. Herbert2, Y. Rosenthal3, J. Kalansky3 # B43C-0412 1) Introduction The Peru-Chile suboxic zone is one of 3 major open ocean regions where vanishingly small subsurface O2 concentrations enable microbially-mediated fixed N loss. This loss associated with denitrification and anammox processes globally is a predominate control on marine N cycling and oceanic N inventory. Past variations in the extent of low O2 and N-loss are recorded in the δ15N of underlying sediments of these regions and prior work has shown climate sensitivity on centennial to orbital time scales. Amongst a number of findings has been a sharp and early rise in δ15N at the beginning of the last deglaciation. Outstanding questions include the nature of the forcing of this rapid deglacial increase in N loss and the relative contributions of imported vs. system generated signals. Of the latter, changes in surface NO3- utilization have the potential to also contribute to the sediment δ15N record. A. B. Figure 3. (A)Age models for CDH-23 & 26 based on radiocarbon dating of planktonic foraminifera. Calendar ages were derived after subtraction of a constant 14C reservoir age effect (~700 yr) and calibration using InterCal 4.0 High and continuous sedimentation is evident for both cores. (B) Benthic foramd18O vs. age for each core in comparison to the Epica Antarctic ice core record. Both the quality of the age model and the time periods covered by each core are evident. 2) Core Location and Regional Context To address these issues, δ15N records were constructed from two long pistons cores recently collected on the northern Peru margin in the vicinity of 4°S (Fig. 1). CDH-23 and CDH-26 were raised from 350 and 1000 m depth. These sites were chosen in expectation of a) high continuous sedimentation rates from the LGM to present and b) overlying water column characteristics are representative of the primary source water for the Peru-Chile OMZ. The primary subsurface water mass constituting the Peru-Chile suboxic zone is sourced in the equatorial undercurrent (‘13°C water’) and enters this system through the Peru-Chile Undercurrent (Fig. 2). From north to south along its flow path, O2 decreases along the margin and reaches levels sufficient to enable subsurface N-loss in the vicinity of 7 to 10° S as indicated by the appearance of NO2- and N deficits (negative N’) and increasing δ15NO3-. CDH-23 and CDH-26 at 4°S are thus located just upstream of the low O2, N-loss region with cores sites we have previously studied within a N-S gradient of increasing OMZ and N-loss intensity. 4) CDH-23 & 26 d15N Modest variations in sediment δ15N were observed over the last 25 kyr. Where CDH-23 & 26 overlap (last 14 kyr), the records are practically identical supporting their fidelity in reflecting near-surface conditions overlying these two nearby cores. The most prominent feature is a 1‰ increase between 18 and 14 kyr followed by a near-steady decrease of 1.5‰ to the late Holocene. Core top values of 5‰ appear to represent the modern δ15N average, but EUC source waters actually have a δ15NO3- of ~6‰. The difference is likely due to HNLC conditions at the site of CDH-23 & 26. PUC Figure 4. Sediment d15N records for CDH-23 & 26. 5) Comparison to OMZ d15N Records Within the Peru-Chile OMZ, margin sites between 9 and 30° S are marked by a large and sharp rise in δ15N. The increase is from 4 to 6‰ and takes place, at most over 2 kyr. The CDH 23 & 26 cores, in contrast, have a deglacial δ15N increase that is a fraction of this magnitude and takes twice as long. The high temporal resolution of these cores indicates that record quality cannot explain the difference. We do note that both cores see subsequent decreases in δ15N into Holocene though there are also clear differences. Our major conclusion is that the CDH 23 & 26 reflect changes in the δ15NO3- of the system and/or local HNLC conditions. Thus most of the δ15N signal found within the OMZ is generated by changes in OMZ intensity and corresponding N-loss. The early rapid rise in OMZ δ15N thus represents a corresponding rapid increase in N-loss. Figure 2. Biogeochemical maps of the Peru OMZ from January and February of 2009. Properties are shown along a constant density surface (σθ = 26.3 kg m-3) within the upper portion of the OMZ corresponding to a depth range of 100 to 170 m (Altabet et al., 2012, Biogeosciences). (A) O2 concentration (µmol kg-1), station locations (B) NO2- concentration (µmol kg-1). (C) Nitrogen anomaly – N’ (µmol kg-1) calculated as [NO3-] + [NO2-] – 16 x [PO4-3]. (D) The δ15N of NO3-. When southward intensification of suboxic conditions reaches [O2]<3 µmol kg-1, the onset of N-loss processes is evident. CDH 23 & 26 3) Dating and Age Models Unlike cores to the south along the Peru margin (Fig. 1), forams were sufficiently abundant for radiocarbon dating in both CDH-23 & 26. This is likely a consequence that, just outside of the OMZ, bottom water conditions along the margin remain conducive to carbonate preservation. Numerous 14C dates were obtained on both cores to achieve well resolved age models (Fig. 3). Accumulation rates were high, ranging from 0.5 to 1.5 m/kyr. Also in contrast to previously studied margin cores, sedimentation was continuous. Whereas shallower CDH-23 reached to the middle of the last deglaciation, CDH-26 spanned the LGM to late Holocene. Benthic foramd18O confirms these age assignments. The offset between cores reflects the warmer overlying water for shallower CDH-23. Figure 5. Sediment d15N records for cores with the OMZ along the Peru margin (Fig. 1). GeoB 7139 is at 30°S and not shown on Fig 1. These data were previously published by De Pol-Holtz (2006, Paleoceanography) Figure 1. Locations of cores discussed in this poster. Acknowledgments: Funding from the NSF P2C2 program, Jen Larkum and Rehka Singh for technical support