The Late Cretaceous- Macleod et al. (2011)

Continuing our understanding of past ocean circulation, we move to the Late Cretaceous. This greenhouse interval can serve as a useful analogue for predicting future climate change. Macleod et al. (2011) utilise an innovative technique for reconstructing past circulation patterns.  They use neodymium (Nd) which in contrast to traditional proxies as discussed in post 2 (e.g. benthic ¹⁸ O), directly tracks water masses as the ¹⁴³Nd/¹⁴⁴Nd ratio of seawater (expressed as ƹ_Nd) is used as a tracer. The authors present ƹ_Nd measurements from Cretaceous to Palaeogene sediments from four cores in the North Atlantic on the Demerara rise of the Northeast coast of South America (Figure 1.)

Figure 1.  Late Cretaceous temperature and ƹ_Nd records. The grey bars indicate times of correlated shifts in both variables.

Source: Macleod et al. (2011)

From 69 to 62 Myr ago, ƹ_{Nd (t)} shift from -16 to -11 was observed, values which appear to correlate to temperature record found in the North Atlantic during the same time period.

They suggest that this may the possibility of increasing northern-sourced water mass, indicating intensification of deep or intermediate water in the North Atlantic 69 Myr ago. They argue that this emphasised a heat piracy model whereby increased export of cool intermediate or deep waters in the North Atlantic are balanced by increased import of warm water from the South Atlantic. (Figure 2).

Figure 2. Schematic representation of circulation in the Atlantic during the late Maastrichtian. Arrows indicate the circulation described in the heat piracy model.

Source: Macleod et al. (2011)

 It is important to reconcile that the termination of the Late Cretaceous greenhouse climate may have terminated primarily due to volcanic CO₂ forcing and that changes in nature and depth in ocean gateways may have profoundly affected circulation patterns. The shifts in ƹ_Nd  measurements appear tightly correlated with changes in Late Cretaceous climate at a resolution of approximately 10⁶ years.  

The paper has implications for our understanding of the mechanics of ocean circulation in greenhouse gas intervals. This is important to take into account when predicting the impact of future climate change on ocean circulation.