Interpretation of the nitrogen isotopic signal variations in the Mauritanian upwelling with a 2D physical‐biogeochemical model

Abstract

[1] A physical-biogeochemical model is used to simulate the evolution of the d 15 N signal during the last glacial-interglacial transition in sedimentary cores offshore from the Mauritanian upwelling. The biological model is a classical nitrogen-based trophic chain model, which also computes the nitrogen isotope fractionation. The 2D physical primitive equation model simulates the coastal upwelling circulation and is applied for different sea level scenarios. The effect of the sea level rise, inducing the shelf immersion, seems to be a main factor explaining the organic nitrogen flux and isotopic signal variations along the last deglaciation. This effect is modulated by an upwelling seasonality that may have been much longer at the Last Glacial Maximum, around 10-11 months instead of 4-5 months at present. Between 15 and 5.5 kyr ago, 60% of the sedimentary d 15 N variations could be explained by this local shelf immersion effect. This reconstruction also reproduces the strong isotopic fall occurring between 5.5 kyr and the present.

Glacial‐interglacial modulation of the marine nitrogen cycle by high‐latitude O2 supply to the global thermocline

“…Many local processes introduce fractionation of N isotopes from the initial N pool and have been cited as potential contributors to the δ 15 N signal recorded in bottom sediments at a given site. Among these are relative nutrient utilization [ Altabet and Francois , 1994], phytoplankton species effects shown in culture [ Montoya and McCarthy , 1995], trophic exchanges shown in food web studies [ Minagawa and Wada , 1984], sea level effects [ Giraud et al , 2003] and diagenesis [ Freudenthal et al , 2001]. However, it has not been proven that, at a given site, changes in any of these local parameters have large impacts on down core δ 15 N profiles over long time periods; the fact that some records show very little change in δ 15 N over hundreds of thousands of years despite significant potential for local influences [ Kienast , 2000] demonstrates that these processes do not necessarily generate significant variability over time.…”

Commentary on “The Anthropogenic Greenhouse Era Began Thousands of Years Ago”

“…Observations have shown that the Last Glacial Maximum -during which the orbital forcing was very similar to today (Berger, 1978) -is associated with a reduced CO 2 concentration (Petit et al, 1999), a substantial cooling of the ocean surface (Sarnthein et al, 2003), a reduction of the ventilation of the North Atlantic deep Ocean (Duplessy et al, 1988), absence of boreal forests (Bigelow et al, 2003) and large permafrost areas (implying frozen wetlands) (Renssen and Vandenberghe, 2003;Kondratjeva et al, 1993). Computer experiments have also shown that large ice sheets modify the dynamics of the atmosphere considerably (Kageyama et al, 1999); the sea-level drop impacts on the primary productivity of coastal ecosystems (Giraud et al, 2003) as well as on coral reefs (Ridgwell et al, 2003). There are thus many sources of interaction between the carbon cycle and ice volume.…”

Modelling an alkenone-like proxy record in the NW African upwelling

“…The stability of the U K 37 signal in the water and sedimentary diagenesis are still questions. Some studies argue for possible modification of the original temperature signal (Hoefs et al, 1998;Gong and Hollander, 1999;Rontani et al, 2005), whereas other studies conclude that there is no diagenetic effect on the temperature record (e.g. Grimalt et al, 2000;Harvey, 2000).…”

“…Table 2 gives a selection of these parameters from other PFT biogeochemical models or laboratory experiments. A maximum growth rate of 0.6 d −1 is widely used for phytoplankton in biogeochemistry models (Le Quéré et al, 2005), and is the value used for the initial NPZD model (Oschlies and Garçon, 1999;Giraud et al, 2000Giraud et al, , 2003. The maximum growth rate for coccolithophores is usually lower than that of the diatoms, by about two thirds (Eppley et al, 1969;Chai et al, 2002;Gregg et al, 2003).…”

“…The biogeochemical model used for this study is derived from the NPZD model described in Oschlies and Garçon (1999) and Giraud et al (2000Giraud et al ( , 2003. The total phytoplankton pool has been divided into two plankton functional types (PFTs).…”