Abstract. The last time in Earth's history when high latitudes were warmer than during pre-industrial times was the last interglacial period (LIG, 129–116 ka BP). Since the LIG is the most recent and best documented interglacial, it can provide insights into climate processes in a warmer world. However, some key features of the LIG are not well constrained, notably the oceanic circulation and the global carbon cycle. Here, we use a new database of LIG benthic δ13C to investigate these two aspects. We find that the oceanic mean δ13C was ∼ 0.2 ‰ lower during the LIG (here defined as 125–120 ka BP) when compared to the Holocene (7–2 ka BP). A lower terrestrial carbon content at the LIG than during the Holocene could have led to both lower oceanic δ13C and atmospheric δ13CO2 as observed in paleo-records. However, given the multi-millennial timescale, the lower oceanic δ13C most likely reflects a long-term imbalance between weathering and burial of carbon. The δ13C distribution in the Atlantic Ocean suggests no significant difference in the latitudinal and depth extent of North Atlantic Deep Water (NADW) between the LIG and the Holocene. Furthermore, the data suggest that the multi-millennial mean NADW transport was similar between these two time periods.
Benthic 13 C is often used to infer past changes in ocean circulation, though the interpretation of this proxy is difficult due to data scarcity and uncertainties. We present two methods for reconstructing the 13 C signal of North Atlantic Deep Water (NADW) and Antarctic Bottom Water and calculating the average oceanic 13 C values for the Atlantic Ocean based on 13 C from benthic foraminifera. The two simple statistical models are described and tested for the Holocene and the Last Glacial Maximum. The first statistical model consists of regressions of the 13 C data, which vary quadratically with depth and linearly with latitude. It differentiates between two regions, one for NADW and another for Antarctic Bottom Water. The second method consists of a hyperbolic tangent regression, which is bound asymptotically by the water mass source region averages (end-members). To test the robustness of the statistical models, two isotope-enabled climate models, the UVic ESCM and LOVECLIM, are sampled randomly, generating "pseudoproxies." These are then used for testing the accuracy of the statistical models against the complete climate model 13 C outputs. We quantitatively compare the average 13 C and NADW depth against the original climate model outputs. We find that both statistical approaches are robust, regardless of the spatial distribution of the pseudoproxies, with the quadratic approach better able to capture the shape of NADW 13 C signal. Hence, this method can potentially be applied to different 13 C data sets to evaluate past changes in NADW.
Abstract. The last time in Earth’s history when the high latitudes were warmer than during pre-industrial times was the last interglacial (LIG, 129–116 ka BP). Since the LIG is the most recent and best documented warm time period, it can provide insights into climate processes in a warmer world. However, some key features of the LIG are not well constrained, notably the oceanic circulation and the global carbon cycle. Here, we use a new database of LIG benthic 𝛿13C to investigate these two aspects. We find that the oceanic mean 𝛿13C was ~ 0.2 ‰ lower during the LIG (here defined as 125–120 ka BP) when compared to the mid-Holocene (7–4 ka BP). As the LIG was slightly warmer than the Holocene, it is possible that terrestrial carbon was lower, which would have led to both a lower oceanic 𝛿13C and atmospheric 𝛿13CO2 as observed in paleo-records. However, given the multi-millennial timescale, the lower oceanic 𝛿13C most likely reflects a long-term imbalance between weathering and burial of carbon. The 𝛿13C distribution in the Atlantic Ocean suggests no significant difference in the latitudinal and depth extent of North Atlantic Deep Water (NADW) between the LIG and the mid-Holocene. Furthermore, the data suggests that the multi-millennial mean NADW transport was similar between these two time periods.
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