کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
4535524 1326114 2008 25 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Age spectra in North Atlantic Deep Water along the South American continental slope, 10°N–30°S, based on tracer observations
موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات زمین شناسی
پیش نمایش صفحه اول مقاله
Age spectra in North Atlantic Deep Water along the South American continental slope, 10°N–30°S, based on tracer observations
چکیده انگلیسی

An extensive set of transient-tracer observations has been used to determine age spectra in Upper and Lower North Atlantic Deep Water (UNADW, LNADW) along the western continental margin between ∼10°N and 30°S. The tracers are CFC-11, CFC-12, CCl4, and tritium, and the observations cover 20 years. The procedure was to deduce from the data the free parameters of a transit time distribution (TTD; mean age or transit time τ, age dispersion σ, and dilution factor by tracer-free water f) of the common functional form (Inverse Gaussian). A novel northern boundary condition for the tracers was employed; it allows for enhanced halocarbon input during convective deep-water formation in winter and yields minimized biases between observed and TTD-based tracer concentrations in downstream NADW. We found that the common boundary condition of time-invariant apparent saturations gave inconsistent results. The TTD parameter fits covered the entire region and used all tracer data jointly, the parameters being prescribed to increase linearly along-stream and the Péclét number (Pe=2τ2σ−2) to be constant. For the southward transit up to the equator, we obtained for UNADW τ=54 years, σ=33 years, and f=1.4, with uncertainties of about ±20% estimated on the basis of a thorough error analysis. A limiting factor was a significant scatter in the NADW tracer fields. The LNADW parameter values are similar but less well constrained than for UNADW. The age spectra are broad; they correspond to large-scale along-core diffusivities on the order of 104 m2/s. The TTD-based mean NADW flow velocities are less than 1 cm/s. The relative southward increases of the TTD-based ages exceed those of CFC-11 to CFC-12 ratio ages about three-fold. An additional result is an improved CCl4 decomposition rate of 0.011±0.002 year−1 at UNADW temperatures. Our TTDs can be used to estimate the NADW-related transfer of dissolved constituents. We present such estimates for 3He generated by tritium decay from the pre-nuclear period onward and for anthropogenic dissolved inorganic carbon (DIC) as examples. The latter estimate yields a NADW-related transport of anthropogenic DIC southward across the equator of about 50 Mt in 1994. A critical aspect of the TTD methodology is that the older part of the age spectra is essentially unconstrained by observations. The TTDs can furthermore be used as a diagnostic tool to check certain aspects of NADW transfer in numerical ocean circulation models.

ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Deep Sea Research Part I: Oceanographic Research Papers - Volume 55, Issue 10, October 2008, Pages 1252–1276
نویسندگان
, , ,