The Azolla event problem. Paleoclimate falsified?

Azolla is a tiny fern genus that floats on fresh water mostly in moderate to warm climates. A big surprise was that Azolla remains were found in sediment cores taken on the Lomonosov Ridge in the Arctic sea in 2004. This became known as the Azolla event

Obviously this Azolla bloom event was subject to intense study, followed by several publications like Speelman et al 2009

ABSTRACT

Enormous quantities of the free-floating freshwater fern Azolla grew and reproduced in situ in the Arctic Ocean during the middle Eocene, as was demonstrated by microscopic analysis of microlaminated sediments recovered from the Lomonosov Ridge during Integrated Ocean Drilling Program (IODP) Expedition 302. The timing of the Azolla phase (~48.5 Ma) coincides with the earliest signs of onset of the transition from a greenhouse towards the modern icehouse Earth. The sustained growth of Azolla, currently ranking among the fastest growing plants on Earth, in a major anoxic oceanic basin may have contributed to decreasing atmospheric pCO2 levels via burial of Azolla-derived organic matter. The consequences of these enormous Azolla blooms for regional and global nutrient and carbon cycles are still largely unknown. Cultivation experiments have been set up to investigate the influence of elevated pCO2 on Azolla growth, showing a marked increase in Azolla productivity under elevated (760 and 1910 ppm) pCO2 conditions. The combined results of organic carbon, sulphur, nitrogen content and 15N and 13C measurements of sediments from the Azolla interval illustrate the potential contribution of nitrogen fixation in a euxinic stratified Eocene Arctic. Flux calculations were used to quantitatively reconstruct the potential storage of carbon (0.9–3.5 1018 gC) in the Arctic during the Azolla interval. It is estimated that storing 0.9 1018 to 3.5 1018 g carbon would result in a 55 to 470 ppm drawdown of pCO2 under Eocene conditions, indicating that the Arctic Azolla blooms may have had a significant effect on global atmospheric pCO2 levels through enhanced burial of organic matter.

Bold mine

I’m interested in the timing. Speelman et al refer to this as follows:

In effect, around this time (~48.5 Ma) the transition from a global greenhouse climate towards the modern icehouse started (Tripati et al., 2005; Zachos et al., 2008), possibly heralded by decreasing atmospheric CO2 concentrations (Pearson & Palmer, 2000; Pagani et al., 2005). Together these notions suggest that sustained growth of Azolla in a major anoxic oceanic basin may have contributed substantially to decreasing atmospheric pCO2-levels.

But what does Pearson & Palmer, 2000 have to say about this timing?

Notice their figure 4:

At first we notice that the benthic d18O starts to decrease around 49Ma indeed. So that cooling part is covered, provided that the isotope proxyis correct- but that’s a different story. However, there are problems with the atmospheric pCO₂ proxies.

Note that the big reduction from about 3600 ppm to 650ppm happened at about 53-52Ma, about 4 Ma year too early. The authors remark about this:

“We note that the termination of North Atlantic volcanism at about 54±53 Myr ago corresponds approximately to the initial drop that we record in pCO2”

Also notice the atmospheric pCO₂ spike from 400 to 2400 ppm at about 46Ma, which coincides with a strong early to mid Eocene cooling according to the Benthic d18O proxy. The authors remark:

Our data do not support a precise covariation of pCO2 and temperature; indeed we record a pCO2 peak during the cooling phase at approximately 45.5 Myr ago.

That should have been a bombshell. But of course, these paleo climate reconstructions from noisy proxies are pretty tricky and can easily be hypothesed away. So are there any other reconstructions that contradict or support this? Let’s have a look at fig 4 of Zachos et al 2008

From the caption:

Figure 2 | Evolution of atmospheric CO2 levels and global climate over the past 65 million years. a, Cenozoic pCO2 for the period 0 to 65 million years ago. Data are a compilation of marine (see ref. 5 for original sources) and lacustrine²⁴ proxy records (…)
b, The climate for the same period (0 to 65 million years ago). The climate curve is a stacked deep-sea benthic foraminiferal oxygen-isotope curve based on records from Deep Sea Drilling Project and Ocean Drilling Program sites⁶…

Noto that the references and sources are different from Pearson & Palmer 2000 however we see the same spikes back albeit maybe a bit younger around 52Ma and 46Ma and again we see no effect from the latter spike on the d18O temperature proxy which would support Pearson & Palmers observation:

…data do not support a precise covariation of pCO2 and temperature

Note also that it’s hard to see any influence of the Azolla bloom event at about 48.5Ma on the ongoing d18O cooling trend after about 52Ma in this fig 2b of Zachos et al 2008.